What is a "framebuffer" and what does it do?

Thank you

It is a method of writing to video graphics display memory directly on the host hardware. This is in contrast to the use of an X server, where applications use the X protocol standard, and an X server does the writing on behalf of the client application. X will work nicely across a network. Framebuffer access is only done on the local host, and typically only by one application at a time. Since framebuffers work by directly exposing video memory, they are fast. Since X works by adhering to a mature standard, it is portable. Framebuffers don't provide any windowing or other drawing services, so applications that use it have a lot more work to do than if they used X. Some applications (games) don't need that, and see speed as the principle measure of performance.
Just to add a twist, an X server may use a framebuffer as its way of performing the actual rendering of graphical output.

--- rod.

One common use of a framebuffer is to display graphical media on console-only systems. It allows you to do things like view an image or watch a video without the need of a running X-server.