The batter is a at-bat and makes contact with the pitch. The baseball goes soaring into the air. As a fielder, you need to be ready to catch it. Fly balls are baseballs that are hit high into the air, and usually travel a far distance, often into the outfield. As the baseball is in the air, fielders try to position themselves around where they think the baseball will land, then catch the baseball as it comes down.
A pop fly is similar to a fly ball, but pop flys tend to go very high into the air, sometimes even clearing the upper deck of the stadium! However, unlike fly balls, they do not travel very far across the field; they usually have the appearance of going straight up and straight back down. Infielders are usually the ones to catch pop flys.
The runners may try to advance at their own risk whether or not the baseball is caught. However, if the baseball is caught, they must tag up first.
Sacrifice plays are any time a batter hits the ball and is subsequently thrown out, but runners already on base advance or score because of the fielders being occupied with throwing the batter-runner out.
Sacrifice plays are always used when there are less than two outs, and there is at least one runner in scoring position (on either second or third base). They are a good way to acquire runs without needing an exceptional hit.
A sacrifice fly (often called a sac fly) is slightly different from a sacrifice bunt, and is a more common occurrence. Unlike a sacrifice bunt, which is used to advance runners, the purpose of a sacrifice fly is to advance a runner to home plate to score (this is also called driving a runner home).
A sacrifice fly goes as follows: A batter hits the baseball into the outfield, or foul territory outside the outfield. A fielder catches the baseball, putting the batter out. Once the baseball has been caught, a runner (usually at second base or third base) runs to home plate while the fielder with the baseball throws the baseball to the catcher so he can put the runner out at home plate. If the runner safely reaches home plate before the catcher has the baseball/can put him out, he scores. Other runners on base can also advance, as long as they are not put out. When a player hits a sacrifice fly, he is not credited with an at-bat. Rather, it counts as a plate appearance.