In the UK odds are usually expressed in one of two formats, the traditional type is fractional odds; these are shown in the form x/y where x is the profit from any given stake y. So odds of 5/1 (5 to 1) will result in a profit of £50 for a £10 bet (a bet of £10 will return £60 as your stake will also be returned). The alternative is to use decimal odds; these are shown as a single figure x which can be multiplied by your stake to give you the return should it win. The equivalent of 5/1 in fractional odds is 6.0 in decimal odds. Decimal odds are generally more useful for the modern gambler as comparisons are easier to make and accumulators (multiple bets) easier to calculate.
Converting between fractional and decimal odds
To convert from fractional to decimal odds simply divide x by y (on the fractional odds) and add one. So 9/4 in fractional odds would be (9/4)+1=3.25.
To convert from decimal to fractional reverse the procedure (take off one and find the lowest proper fraction).
for an odds conversion table.
When the probability that the event will not happen is greater than the probability that it will, then the odds are "against" that event happening. Odds of, for example, 6/1 (6 to 1) can be said to be "6 to 1 against". In decimal odds this would be 7.0.
"Odds on" is the opposite of "odds against". It means that the event is more likely to happen than not. This is sometimes expressed with the smaller number first 1/4(1 to 4) but often using the word "on" ("4 to 1 on") For example, on a £4 bet, you will be given £1 plus the returned stake of £4, giving a £1 profit. In decimal odds this would be 1.25.
"Even odds" occur when the probability of an event happening is exactly the same as it not happening. In common parlance, this is a "50-50 chance". Guessing heads or tails on a coin toss is an example of an event that has even odds. It is commonly referred to as "even money" or simply "evens" 1/1(1 to 1, or 2.0 in decimal odds).
Odds are not static things, particularly close to the start of an event they will move up (drift) and move down (tighten) dependent on where the money is being placed by punters around the world. Bookmakers will adjust their books to try and make sure they make a profit no matter what the result. Betting exchanges fluctuate in the same way as a stock exchange might, although often more quickly as events change the chances of any particular result occurring.