The betting table has inner and outer tracks, which has led to wagers being known as Inside and Outside Bets. You're a lot more likely to win Outside Bets, because they have an almost 50% chance of occurring.
With Outside Bets, you'll be predicting whether the ball will land in an odd- or even-numbered panel, a red or black panel, or a panel numbered from 1 to 18 or from 19 to 36. The House Edge comes in the fact that if the ball finishes up in a green panel, you lose automatically.
Inside Bets are on groups of particular numbers, and can get as specific as predicting the exact panel that the ball will hit. They carry higher odds, and larger rewards.