Summarize the following:
A bookie sets the Over/Under, and you simply choose if the combined score will be higher or lower than that mark. Over/Under bets are often less risky and complicated than other forms of betting.  If the O/U is 198.5 points and you bet over, then any combined score from 199 points and up will pay out. If the score is exactly what the bookies set, then the bet is a push and everyone gets their money back. Make sure to check this with your bookie first, however.
Know that Over/Under bets are placed on the combined score of both teams and are much simpler to place. Most payouts for Over/Under bets are even, meaning that if you wager $100 you win $100 profit.