This is really ungrateful question to Google.
Basically, I would like to know, regarding openGL, what are various methods to do some kind of computation in one of the shaders and have the results stored in VRAM to be used and accessed at a later time?
Some helpful google-able terms will include:
render to a texture use framebuffer object read from a texture using glReadPixels
Hopefully by "computation" you mean "drawing" of some kind. Hope that helps!
http://www.opengl-tutorial.org/intermediate-tutorials/tutorial-14-render-to-texture/
https://stackoverflow.com/questions/32149512/opengl-how-to-read-back-texture-buffer
