While you can query the location of a generic vertex attribute in a GLSL shader by calling glGetAttribLocation(GLuint program, const GLchar *name), you can also set the location of a generic vertex attribute manually using glBindAttribLocation(GLuint program, GLuint index, const GLchar *name)
NOTES: Manually set attribute locations do not take effect until glLinkProgram(GLuint program) is called.
You cannot manually assign a location to a built in vertex attribute (e.g. gl_Vertex).
It is possible to assign the same location to multiple attributes. This process is known as aliasing, and is only allowed if just one of the aliased attributes is active in the executable program. HOWEVER the implementation is not required to check for aliasing and is free to employ optimizations that only work in the abscence of aliasing.
Any vertex attribute which is not manually assigned a location will be assigned one by the linker, and this location can be queried with glGetAttribLocation(GLuint program, const GLchar *name).
glBindAttribLocation man page