Difference between GL_CLAMP and GL_CLAMP_TO_EDGE

[apapaxionga]

what is the difference between GL_CLAMP and GL_CLAMP_TO_EDGE?

[ZbuffeR]

If you ask, then just use GL_CLAMP_TO_EDGE.
The difference is that GL_CLAMP works correctly provided you have correctly set up a "texture border", otherwise you will see a blackish outline (default texture border color) around a textured quad with 0..1 texcoords. GL_CLAMP_TO_EDGE do no involve texture border.
Anyway this is detailed in the spec if you really want to know the crispy details.