The ball is live when the down begins. A live ball is any ball that is being played and remains in bounds. The ball is live for the entirety of a play until it goes out of bounds or until an official stops the play. After every touchdown the ball is dead until the game restarts.
Players from both teams line up on the scrimmage line. As soon as the snapper snaps the ball, the ball is considered live and in play. Any time players line up on the line of scrimmage, it is because they are putting the ball back into play again.
Before the play clock goes to 0:00, the offense must snap the ball to begin the down. A snap is a backward pass throw the legs from the center to the quarterback (both are players on the offense). Scrimmage downs always begin with a snap, also known as a hike.
The ball is live when the snap is made.
When the ball is dead, the play has stopped and no one will be chasing after the ball to score.
At the end of every down a group of referees called the chain crew mark the scrimmage line, first down line, and the distance between the two lines on the field. The chain crew and all seven officials are the ones responsible for calling when the ball is dead and where the line of scrimmage is for that play.