RCA is only a type of cable/jack. It can be used for composite or component:
Component cable can output YPbPr or RGB video signal.
So RGB is a video signal, traditionally obtained (for the last 20 years in Europe) through a scart cable:
RGB/scart is usually considered the best quality a retro system can give. This includes Master System, Mega Drive, Saturn, SNES... even Dreamcast, PS2 and Xbox if connected to a CRT. If connected to a plasma/LCD, Dreamcast can use VGA, while PS2 and Xbox can use component. Dreamcast will most definitely look the best of the tree on a plasma/LCD if using VGA.
Uh... sorry for my english