Grant already posted the link to my "buying bikes for commuting off craigslist" post for me!
http://biodieselhauling.blogspot.com/2012/01/buying-bikes-from-craigslist.htmlAnd everyone covered just about everything.
I just wanted to answer one of your questions: no, a commuter bike and a hybrid are not the same thing.
A hybrid is specifically a cross between a mountain bike and a road bike. It has flat handlebars, usually much more upright than a normal road or mountain bike, moderately wide, smooth, 700C wheels (road bike standard size), wide range of gears. It is decent on the road, ok for very mild off-road, but not nearly as good as a mountain bike off road or as fast as a road bike on road.
A commuter bike could be lots of different things. It isn't an official class of bike. Something sold specifically as a commuter probably comes standard with racks and fenders, perhaps internal gears or a built in headlight or a step-through frame - but you will pay a premium for it over just getting a regular bike and adding your own racks, fenders and lights. Many (but not all) commuter bikes will be hybrids. Many hybrids would make a good commuter bike. But ultimately, a commuter bike is just something that someone uses to commute. Its like a "family car".
I found I was able to do my commute substantially faster on my racing bike than my touring bike, and they were both road bikes.