The first dozen covers numbers 1-12, the second covers 13-24, and the third covers 25-36. On a table, you'll find this bet located in-between the even money bets and the inside numbers. Just like a column bet, a single dozen will payout at 2 to 1.
Say a single-number bet has a 35:1 payout, and you betR$1. If you win, you'll get your money back plusR$35. Another example: If you win aR$100 bet (for a 35:1 payout), you get a total of $3,600 ($3,500 profit plus theR$100 you initially bet).