The 0 is one of the 37 numbers (in single -zero roulette). The casino has an edge because it cheats you when you win, by paying you less than it should. But losing is free!
From "The Roulette Winner" by Lee Tutor:
..."The casino edge, or house advantage, is “the
difference between what the casino should pay you,
and what it actually does pay you, when you win”.
Let’s look at the simple mathematics involved in
roulette payouts: Because there are thirty seven
numbers on the European single zero wheel, there is a
one in thirty seven chance, (1/37), to win a “straight
up” bet. It follows that on a win you should get paid at
36 to 1 when you win your bet. (36 to 1, means that
for every unit that you wager, when you win you
should get 36 units profit plus your original unit back
for a total of 37 units.)
These are the “true odds” but not what you get paid
at the casino! If you make a one unit straight up bet
and win it, the casino will only pay you at 35 to 1,
instead of 36 to 1, and so it has short-changed you by
one unit, which means that 1/37th of your winnings
have gone to the house.
So, every time you win a bet, the casino also wins
something back from you too! The house advantage
for a single zero wheel can be worked out by the
following formula:
(36- 35) x 1/37= 1/37
In percentage terms that is 1/37 x 100 = 2.7027%, or
2.7% approximately....
In the US, the casino advantage on roulette is even
higher. There, the wheel has 38 numbers, because it
has an extra zero, and the casino short-changes you by
2/38ths, because if you were to bet one unit on a
straight up number and you won, you’d only be paid at
35 to 1 instead of 37 to 1. So in effect you'd lose two
whole units of profit, as opposed to only losing the one
unit that you lose on a single-zero wheel.
Always remember that in the US, because of the
double-zero wheel with 38 numbers, the casino edge is
5.26%. Worse still on a “corner bet”, which covers the
five numbers: 0, 00, 1, 2, 3, the house edge rises
further still to 7.89%, because they only pay at 6:1 for
a five number bet (instead of 6.6:1). Avoid double-zero
wheels, except if there is a “surrender rule”, in which
case only bet on the even chance bets, like red/black,
Even/Odd, high/low...."