OpenGL explained: Framebuffer Object & Renderbuffer

Upon looking around for some info on an issue I'm currently having, I found the following posted by user "Nicol Bolas" on StackOverflow. I've read a few of this user's posts and they are quite knowledgeable on OpenGL related things.

Also, Nicol Bolas is the name of the chaos dragon from Magic The Gathering. Coincidence? Lol.

Anyway, the description is quite concise in my opinion.

A framebuffer object is a place to stick images so that you can render to them. Color buffers, depth buffers, etc all go into a framebuffer object.
A renderbuffer is like a texture, but with two important differences:

1. It is always 2D and has no mipmaps. So it's always exactly 1 image.

1. You cannot read from a renderbuffer. You can attach them to an FBO and render to them, but you can't sample from them with a texture access or something.

So you're talking about two mostly separate concepts. Renderbuffers do not have to be "for depth testing." That is a common use case for renderbuffers, because if you're rendering the colors to a texture, you usually don't care about the depth. You need a depth because you need depth testing for hidden-surface removal. But you don't need to sample from that depth. So instead of making a depth texture, you make a depth renderbuffer.
But renderbuffers can also use colors rather than depth formats. You just can't attach them as textures. You can still blit from/to them, and you can still read them back with glReadPixels. You just can't read from them in a shader.

http://stackoverflow.com/questions/9850803/glsl-renderbuffer-really-required

So you can't really use a renderbuffer for anything yourself. You put it in there so the system has a place to write things.

The framebuffers are for things you plan to work with.