Last week, the CBC staff gathered before the shop opened for a mechanical tutorial -- the subject was bottom brackets (around which the pedals turn), but here's a little introduction to rotational systems in general.
A "rotational system" is any part of the bike that involves circular motion -- the wheels (hubs), steering (headset), and the drive-train (bottom bracket). Each of these parts of the bike operates in a similar way, containing a set of parts that work together to allow smooth, but solid, motion.
Perhaps the most important part of rotational system is the bearings, small metal balls that reduce friction by distributing the pressure placed upon them. Before they were invented, bikes weren't nearly as smooth or comfortable to ride, in fact they were called "boneshakers" and looked like this -- ouch!
For more information about rotational systems (specifically bottom brackets), check out chapter 3 of the Bikes Not Bombs training manual. Jim Langley has a great page that offers a step-by-step guide to overhauling hubs.