Inside bets are made on the individual numbers, one through 36, and the zeros. Say you bet on number 23. If the ball drops into the number 23 pocket, you're paid off at 35-1. To bet on two numbers, place your chips on the line between the two numbers you wish to bet on, such as between 1 and 2, or 1 and the single zero, or 1 and 4. Those are all the inside bets.
Outside bets are the wagers made on the outside of the layout.
If you place your chips in the even box and the ball drops into the number 24, you win. If it drops into number 23, you lose. Simple as that. These bets pay even money. The first 12, second 12 and third 12 all pay 2-1.