When the future is in danger, George Carlin is sent back in time to ensure that Bill and Ted aren't separated due to failing grades. By giving the boys a time machine, they are able to travel through time to learn about the history they'd ignored. On the way they gather Napoleon Bonaparte, Billy the Kid, Socrates, Sigmund Freud, Ludwig Von Beethoven, Joan of Arc, Genghis Khan, and Abraham Lincoln.
Unlike the Back to the Future trilogy, the rules of time travel are a little different in Bill & Ted. Whereas in Back to the Future you can go to any time, you are transported to the same place as you left. In Bill & Ted you can travel not only to any time, but also any place. Bill & Ted also has an odd rule where time is always moving forward in the future putting a bit of a pinch on Bill and Ted's traveling. This seems to imply that time is cyclical, and that in order to travel to certain places certain other conditions must also be met. It's never quite spelled out though, but perhaps the fact that their time travel machine is from the distant future explains it's difference from Doc Brown's.
Ted: Party on, dudes!
1. I think The Matrix largely did this, though part of the joy of The Matrix trilogy comes from seeing Keanu Reeves in a role that didn't seem to fit.
3. Of course, we can't forget his earlier performance in The Lost Boys as a random vampire who wasn't Kiefer Sutherland.