Erlangen program
The Erlangen program is a method of characterizing geometries based on group theory and projective geometry. It was published by Felix Klein in 1872 as Vergleichende Betrachtungen über neuere geometrische Forschungen. It is named after the University ErlangenNürnberg, where Klein worked.
By 1872, nonEuclidean geometries had emerged, but without a way to determine their hierarchy and relationships. Klein's method was fundamentally innovative in three ways:

 Projective geometry was emphasized as the unifying frame for all other geometries considered by him. In particular, Euclidean geometry was more restrictive than affine geometry, which in turn is more restrictive than projective geometry.

 Klein proposed that group theory, a branch of mathematics that uses algebraic methods to abstract the idea of symmetry, was the most useful way of organizing geometrical knowledge; at the time it had already been introduced into the theory of equations in the form of Galois theory.

 Klein made much more explicit the idea that each geometrical language had its own, appropriate concepts, thus for example projective geometry rightly talked about conic sections, but not about circles or angles because those notions were not invariant under projective transformations (something familiar in geometrical perspective). The way the multiple languages of geometry then came back together could be explained by the way subgroups of a symmetry group related to each other.
Later, Élie Cartan generalized Klein's homogeneous model spaces to Cartan connections on certain principal bundles, which generalized Riemannian geometry.
The problems of nineteenth century geometry
Since Euclid, geometry had meant the geometry of Euclidean space of two dimensions (plane geometry) or of three dimensions (solid geometry). In the first half of the nineteenth century there had been several developments complicating the picture. Mathematical applications required geometry of four or more dimensions; the close scrutiny of the foundations of the traditional Euclidean geometry had revealed the independence of the parallel postulate from the others, and nonEuclidean geometry had been born. Klein proposed an idea that all these new geometries are just special cases of the projective geometry, as already developed by Poncelet, Möbius, Cayley and others. Klein also strongly suggested to mathematical physicists that even a moderate cultivation of the projective purview might bring substantial benefits to them.
With every geometry, Klein associated an underlying group of symmetries. The hierarchy of geometries is thus mathematically represented as a hierarchy of these groups, and hierarchy of their invariants. For example, lengths, angles and areas are preserved with respect to the Euclidean group of symmetries, while only the incidence structure and the crossratio are preserved under the most general projective transformations. A concept of parallelism, which is preserved in affine geometry, is not meaningful in projective geometry. Then, by abstracting the underlying groups of symmetries from the geometries, the relationships between them can be reestablished at the group level. Since the group of affine geometry is a subgroup of the group of projective geometry, any notion invariant in projective geometry is a priori meaningful in affine geometry; but not the other way round. If you add required symmetries, you have a more powerful theory but fewer concepts and theorems (which will be deeper and more general).
Homogeneous spaces
In other words, the "traditional spaces" are homogeneous spaces; but not for a uniquely determined group. Changing the group changes the appropriate geometric language.
In today's language, the groups concerned in classical geometry are all very well known as Lie groups: the classical groups. The specific relationships are quite simply described, using technical language.
Examples
For example, the group of projective geometry in n dimensions is the symmetry group of ndimensional projective space (the general linear group of degree n + 1, quotiented by scalar matrices). The affine group will be the subgroup respecting (mapping to itself, not fixing pointwise) the chosen hyperplane at infinity. This subgroup has a known structure (semidirect product of the general linear group of degree n with the subgroup of translations). This description then tells us which properties are 'affine'. In Euclidean plane geometry terms, being a parallelogram is affine since affine transformations always take one parallelogram to another one. Being a circle is not affine since an affine shear will take a circle into an ellipse.
To explain accurately the relationship between affine and Euclidean geometry, we now need to pin down the group of Euclidean geometry within the affine group. The Euclidean group is in fact (using the previous description of the affine group) the semidirect product of the orthogonal (rotation and reflection) group with the translations. (See Klein geometry for more details.)
Influence on later work
The longterm effects of the Erlangen program can be seen all over pure mathematics (see tacit use at congruence (geometry), for example); and the idea of transformations and of synthesis using groups of symmetry has become standard in physics.
When topology is routinely described in terms of properties invariant under homeomorphism, one can see the underlying idea in operation. The groups involved will be infinitedimensional in almost all cases – and not Lie groups – but the philosophy is the same. Of course this mostly speaks to the pedagogical influence of Klein. Books such as those by H.S.M. Coxeter routinely used the Erlangen program approach to help 'place' geometries. In pedagogic terms, the program became transformation geometry, a mixed blessing in the sense that it builds on stronger intuitions than the style of Euclid, but is less easily converted into a logical system.
In his book Structuralism (1970) Jean Piaget says, "In the eyes of contemporary structuralist mathematicians, like Bourbaki, the Erlangen Program amounts to only a partial victory for structuralism, since they want to subordinate all mathematics, not just geometry, to the idea of structure."
For a geometry and its group, an element of the group is sometimes called a motion of the geometry. For example, one can learn about the Poincaré halfplane model of hyperbolic geometry through a development based on hyperbolic motions. Such a development enables one to methodically prove the ultraparallel theorem by successive motions.
Abstract returns from the Erlangen program
Quite often, it appears there are two or more distinct geometries with isomorphic automorphism groups. There arises the question of reading the Erlangen program from the abstract group, to the geometry.
One example: oriented (i.e., reflections not included) elliptic geometry (i.e., the surface of an nsphere with opposite points identified) and oriented spherical geometry (the same nonEuclidean geometry, but with opposite points not identified) have isomorphic automorphism group, SO(n+1) for even n. These may appear to be distinct. It turns out, however, that the geometries are very closely related, in a way that can be made precise.
To take another example, elliptic geometries with different radii of curvature have isomorphic automorphism groups. That does not really count as a critique as all such geometries are isomorphic. General Riemannian geometry falls outside the boundaries of the program.
Some further notable examples have come up in physics.
Firstly, ndimensional hyperbolic geometry, ndimensional de Sitter space and (n−1)dimensional inversive geometry all have isomorphic automorphism groups,
the orthochronous Lorentz group, for n ≥ 3. But these are apparently distinct geometries. Here some interesting results enter, from the physics. It has been shown that physics models in each of the three geometries are "dual" for some models.
Again, ndimensional antide Sitter space and (n−1)dimensional conformal space with "Lorentzian" signature (in contrast with conformal space with "Euclidean" signature, which is identical to inversive geometry, for three dimensions or greater) have isomorphic automorphism groups, but are distinct geometries. Once again, there are models in physics with "dualities" between both spaces. See AdS/CFT for more details.
The covering group of SU(2,2) is isomorphic to the covering group of SO(4,2), which is the symmetry group of a 4D conformal Minkowski space and a 5D antide Sitter space and a complex fourdimensional twistor space.
The Erlangen program can therefore still be considered fertile, in relation with dualities in physics.
In the seminal paper which introduced categories Saunders Mac Lane and S. Eilenberg stated: "This may be regarded as a continuation of the Klein Erlanger Programm, in the sense that a geometrical space with its group of transformations is generalized to a category with its algebra of mappings"^{}
Relations of the Erlangen program with work of C. Ehresmann on groupoids in geometry is considered in the article below by Pradines.^{}
The Erlangen Program is carried into mathematical logic by Alfred Tarski in his analysis of propositional truth.^{[]}
References
 Klein, Felix 1872. "A comparative review of recent researches in geometry". Complete English Translation is here http://arxiv.org/abs/0807.3161.
 Sharpe, Richard W. Differential geometry: Cartan's generalization of Klein's Erlangen program. Vol. 166. Springer, 1997.
 Heinrich Guggenheimer (1977) Differential Geometry, Dover, New York, ISBN 0486634337.
 Covers the work of Lie, Klein and Cartan. On p. 139 Guggenheimer sums up the field by noting, "A Klein geometry is the theory of geometric invariants of a transitive transformation group (Erlangen program, 1872)".
 Thomas Hawkins (1984) "The Erlanger Programm of Felix Klein: Reflections on Its Place In the History of Mathematics", Historia Mathematica 11:442–70.
 Hazewinkel, Michiel, ed. (2001), "Erlangen program", Encyclopedia of Mathematics, Springer, ISBN 9781556080104
 Lizhen Ji and Athanase Papadopoulos (ed.) Sophus Lie and Felix Klein: The Erlangen program and its impact in mathematics and physics, IRMA Lectures in Mathematics and Theoretical Physics 23, European Mathematical Society Publishing House, Zürich, 2015.
 Felix Klein, 1872. "Vergleichende Betrachtungen über neuere geometrische Forschungen" ('A comparative review of recent researches in geometry'), Mathematische Annalen, 43 (1893) pp. 63–100 (Also: Gesammelte Abh. Vol. 1, Springer, 1921, pp. 460–497).
 An English translation by Mellen Haskell appeared in Bull. N. Y. Math. Soc 2 (1892–1893): 215–249.
 The original German text of the Erlangen Program can be viewed at the University of Michigan online collection at , and also at [2] in HTML format.
 A central information page on the Erlangen Program maintained by John Baez is at [3].
 Felix Klein, 2004 Elementary Mathematics from an Advanced Standpoint: Geometry, Dover, New York, ISBN 0486434818
 (translation of Elementarmathematik vom höheren Standpunkte aus, Teil II: Geometrie, pub. 1924 by Springer). Has a section on the Erlangen Program.