You got it. Socket A = AMD chips. Durons, Athlon Thunderbird cores, and Athlon XPs all fit in Socket A. Intel Celerons and Pentium 4's fit in Socket 478. The FSB determines which of these processors can fit in their socket. A 200/266 MHz FSB Socket A motherboard will support Durons, Athlons, and Athlon XP's that are 200 MHz and 266 MHz FSB. It'll go up to any processing speed that you want as long as the FSB barrier isn't breached.
Overclocking allows the FSB to be raised higher than you're supposed to be able to, but we'll just keep it to the basics.
-Lasereth