Single:
A Single is a bet on 1 selection in one event. Your selection must be successful to get a return.
Double:
A Double consists of 1 bet made up of 2 selections in different events. Both must be successful to get a return.
Treble:
A Treble consists of 1 bet made up of 3 selections in different events. All must be successful to get a return.
Accumulator:
An accumulator bet is made up of a number of selections (more than 3) one chooses to win from different events. All must be successful to get a return.