Looking for a bike to ride to work?

Now that summer is here, I wanna get a bike to ride to work. Ive been looking on craigslist, but didn't know there were so many kinds. Whats the difference between a road bike, a mountain bike, a cruiser ect. I live in a flat city, and only plan on using it for getting to and from work.... Takin rides in the park... Nothin real serious. Whats the best bike kind for the casual biker?