There are two kinds of lives: single-use hearts and time-limited infinite lives. 

By default, you have 5 hearts (lives). You lose a heart if you fail a level or quit the game without finishing a level. 

If you have an active timer for infinite lives, then you have an unlimited number of lives available for a certain amount of time. In other words, you do not lose a life if you lose a level for that period. The infinite lives timer cannot be paused, so it keeps counting down even if you quit the game.

You need lives to start playing any level in the game. If you fail a level, you lose a life. You also lose a life if you close the game in the middle of a level without finishing it.

If you run out of lives, you can wait until they are restored (1 life takes 30 minutes to be restored), buy extra lives with rubies, or request lives from friends. Here you can read more about purchases, and on this link you can find out how to exchange lives. 

No, if a timer for infinite lives is active, you do not lose hearts when you lose a level. Furthermore, your lives continue to be restored until you have five available even if an infinite lives timer is active.

If this happens, please pay close attention to the specific prize you won. In addition to infinite lives, you may also get a reward with infinite boosters (bombs, lightning, and stars). These are also activated automatically, and their timer is displayed in the overview window that appears before you start a level. 

If you believe that you did receive infinite lives as a reward but do not see evidence that they have been activated, then please check the level overview window. It may well be that you actually received a reward of infinite boosters. 

When you receive infinite lives, they appear in the top right corner of the screen in place of the standard life counter and are marked by an infinity symbol (∞). There is also a timer that indicates the amount of time left before your infinite lives run out. If you won infinite boosters, then the infinity symbol will appear in the level overview window near the icon of the corresponding booster. 

If you still have questions, please contact the support team.