Lives don't play a major role in the game until things start to get difficult. You can see your current quantity of lives by looking in the middle at the top of the screen - the icon indicated by a heart. If you complete a level successfully, you will maintain your number of lives. If you fail a level, you will lose a life. If your Lives reach zero, you cannot play the game until they have recharged.
How to get more lives:
To get more lives you can purchase more using Facebook Credits, or alternatively if you wait for 30 minutes (you can see the time remaining next to the heart on screen), you will receive one back - this is the time they roughly take to recharge. You can have a maximum of five lives. Note that if you quit a level, this also counts as failing.
Although friends can give you lives, once you reach 5 you cannot get anymore, so any extras that you have coming your way would be wasted.
It's important to try not to run out of lives, but you can get more easily enough.