Odds can be expressed in many ways. The most common way of expressing odds is as 'something to something'. 2-1 is read as '2 to 1' and means that for every 1 credit bet, 2 credits of profit will be returned with the original bet if the you win.

Generally, where the odds are expressed as 'a to b', the total return to you, if you win, will be a + b where 'a' is the 'profit' and 'b' is the original bet.

Odds are sometimes also expressed as 'a for b'. If this is the case, for every bet of 'b', a TOTAL amount of 'a' will be returned if you win (this would include your original bet). 

Even money refers to odds of 1-1 where you would receive double your money back if you win.