A bookie is someone who facilitates gambling by setting odds, accepting and placing bets, and then paying out the winnings. It's short for the term bookmaker.
A bookie was a slang term more often used when online sports betting wasn't readily available. Bookies were more popular prior to the internet explosion because they could take bets in person or over the phone, often illegally. Nowadays, a bookmaker is usually part of a sportsbook or casino.
Bookmakers don't bet themselves, as they make money from charging a vigorish, or a transaction fee. The biggest role for bookmakers is to set the odds, often using calculations from a variety of sources, in order to receive equal money on both sides of a bet.
For example, if the bookie opens a spread at Patriots -10 against the Jets and 80% of the money is going to the Patriots, the bookie will likely push the number higher until there is closer to an equal amount of bets on both sides.
While that may never be possible, it's best for the bookie to get equal money on each side because that leads to a higher probability of them winning money. If the money is balanced and there is 50% of bets on each side, the bookie earns the transaction fees and comes out positive. If the spread moved up to -14 in the above situation and 70% of people were still betting the Patriots and they won by 20 points, the bookie would lose money.
Bookmakers can work on their own, but they are often part of a casino. In places like Las Vegas where there are numerous sportsbooks in a small vicinity, bookmakers will often get odds from the same source or base their odds off another location. Now that betting is legal across the country following the repeal of PASPA in 2018, the difference in odds and lines will only grow across states.