Coins are used to buy:

• Extra Moves
• Life Refills
• Boosts
• Keys

Coins can be obtained tapping/clicking on the '+' next to the current Coins total and buying more with real cash.

Note: It's not possible to earn Coins by playing levels and asking friends. 

Coins aren't synced across platforms; i.e. Coins purchased on mobile will not show up on Facebook and vice-versa.