From LQWiki

RFB stands for Remote FrameBuffer. RFB is a protocol for sending graphics from one computer to another. RFB is the basis of Virtual Network Computing (VNC), a cross platform tool for remotely accessing or sharing desktops.

See this link to Cambridge University's website describing the RFB protocol.

KDE's implementation of RFB is currently the only implementation that allows sharing of a desktop via a VNC client, instead of merely allowing remote access.