Bookmakers on the market offer odds in various forms such as fractional, decimal and American odds. Decimal odds and fraction odds carries a mathematical relationship and american odds is the amount required to win 100$. I.E a -200 bet means that you would need to bet 200$ to win 100$, if the odds is positive it´s simply what you would win on a 100$ bet. I.E a +100 bet means that you would win 100$ if you bet 100$. Choose whatever system you´re most familiar with, american oddsformat is good if you want to calculate winnings quickly and fraction/decimal format is good to determine if it´s a mathematical good bet or not (I.E if you consider a team to have a 50% chance of winning it´s much easier to determine if the odds is in your or the bookmakers favors in those formats).
Feel free to use this free tool which is provided by Betsmart to start converting odds on your own.
Odds conversion can be helpfull when you quickly need to compare a play on several different bookmakers. Or when reading sport predictions which specifies their selection in an odds format which you are not used to.
Once you get the hang of it with this tool and the table above you´ll be able to do the conversions in your head with out even thinking about it.