I wrote a little cheat-sheet for data transfer in OpenGL. Here it is. Hope you find it useful!

From\To	Texture	Framebuffer	Host Memory
Texture	Bind texture to framebuffer using glFramebufferTexture	glDraw*	If OES, bind texture to framebuffer using glFramebufferTexture. Else, glGetTexImage
Frame-buffer	glCopyTexImage2D	Multiple render targets?	glReadPixels
Host Memory	glTex(Sub)Image2D. Can use glTexStorage2D to allocate if using new GL	Host memory -> Texture -> Framebuffer	memcpy?