Boosters become available to you as you move up the levels in Candy Crush Saga.

Boosters come in two forms.

The ones that are available before starting a level:

 

Notice that each Booster has a number attached to it that shows how many boosters you have. The grayed ones cannot be used on this particular level. You can select and deselect a booster by clicking on it.

Booster Image Description
This booster adds 3 extra moves. To get this booster – you need to be stuck at a level for more than 2 days and then a friend may help you by sending it to you.
You start the board with one wrapped and one striped candy ( randomly placed).
You start the board with a randomly placed color bomb.
This booster adds 15 extra seconds to the timed level.
Adds “Jelly fish” candies to the candy mix – which clears 3 pieces of jelly when you match 3 candies.
Adds a “Coconut Wheel” to the mix. When you combine a Coconut wheel with any candy- it will roll to its side creating 3 striped candies.

You may also want to check the combinations that can be made using the boosters so you can get the most out of them.

The other type of boosters can be seen to the top right when you start the level.

Booster Image Description
Smashes any square on the grid completely. Use it to either set up a delicious combo – or – most usefully – a particularly stubborn last square of jelly.
Allows you to switch 2 candies that don’t match.
Adds five extra moves to Candy Bombs.
Adds 5 additional moves.
Eats several Licorice X blockers – Chocolate- Licorice Swirls – Meringues and Marmalade.
Sent by friends and adds 3 additional