glTexBuffer is part of the OpenGL 3.1+ standard.
glTexBufferARB is part of OpenGL as an ARB (Architecture Review Board) extension.
glTexBufferEXT is part of OpenGL as a vendor (NVIDIA, ATI etc...) extension (i.e if found you download gl2.h from NVIDIA's website).
So basically, if you use an old version of OpenGL you need to use the ARB or EXT versions.
However, if you use something like libglew, it will match the correct function (pointer) to glTexBuffer so you don't need to worry about what is providing the actual functionality, you just use "normal" OpenGL.
For sample implementations, you might want to look at open-source software implementations of OpenGL (such as mesa http://cgit.freedesktop.org/mesa/mesa/tree/src).