Framebuffer

Framebuffer is not a buffer. It is a collection of states, which has very low memory footprint. Imagine it is a C struct with lots of pointer to a real buffer, etc.

Renderbuffer

Renderbuffer object is newly introduced for offscreen rendering. It allows to render a scene directly to a renderbuffer object, instead of rendering to a texture object. Renderbuffer is simply a data storage object containing a single image of a renderable internal format. It is used to store OpenGL logical buffers that do not have corresponding texture format, such as stencil or depth buffer.