Before understanding the inside and outside bets you need to be clear of the table layout for roulette.

European Roulette Table Layout

The table layout consists of three columns, where the 36 numbers in the roulette wheel are arranged with 12 numbers in each column, there is also a box for the one zero found in the green colored slot.  The numbers in the wheel are with one slot in red and the next in black with the zero in a green pocket.  In the table layout, the red and black colored numbers are marked alternating with each other.  The even and odd numbers are marked in an alternate order.

American Roulette Table Layout

The table layout is similar to the European version with boxes meant for 0 and 00 available in the layout.

Outside Bets

Outside the table layout where the numbers are marked there are boxes that indicate 1^st 12, 2^nd 12, 3^rd 12, 1 to 18, even, odd, red, black, 19 to 36 and 2 to 1 to place the outside bets.

Inside Bets

Those bets that are placed straight in to the three columns are in the point of intersection of two numbered boxes on the layout are meant for inside bets.

The players can place their bets in the numbers in the column, the first twelve, second twelve, or third twelve numbers. Bets can also be placed in numbers 1 to 18 or numbers 19 to 36, in odd numbers or even numbers, in red colored numbers or green colored numbers or black colored numbers, and in 2, 3, 4, or 5 numbers or in a single number as desired.

The special thing about roulette rules for bets is that you can place as many types of bets you want per spin; however, the ball will land on just one number.  You are trying to cover maximum numbers to trap the single number scripted to win.   The combination of bets is all the skill about.