Why are they called "Bookmakers "? Well, bookmakers or bookies, sometimes known as Turf Accountants, are called Bookmakers because they make "books". Not that they are book-writers or book-makers as in book binders. The "book" a company like coral bookmakers make is a book of figures, a mathematical table where the amounts of money which are wagered on different outcomes, potential winners in a horse race or football match for example, and added up in conjunction with odds, so as to calculate new odds to offer.
The idea of the calculations is so that whatever the outcome, the book always adds up, and the bookmakers always makes a profit. Some punters will win and some will lose, but the bookies will almost always win. What's interesting is that because most people bet in a mostly arbitrary way, and bookmakers need to make a profit, it's quite possible to bet in clever ways which allow you to make a profit too. You are winning versus other punters' poor choice, not versus the bookmakers.
The interesting upshot of this is that the odds are very little to do with the actual probability, and are much more of a democracy-based thing, where it's the opinion of the punters and uk bookmakers which shapes the odds. Therefore there exist odds being offered by bookmakers which are much better than the real probability, and which, if you could spot them, you could consistently win a lot of money! What's more, if you did, the bookies would not mind. They don't mind if you win or lose, provided the average win/lose in the punter population leaves them in profit. They call this the bookmakers over-round.
If you decide to put some of this to the test, try out my free betting links for UK bookmakers. I reccommend Coral bookmakers, they offer a free £30 bet when you open an online account at Corals bookmakers.