A kicker has a different goal in mind on different kicks. For example, on field goals and extra points the kicker is aiming to kick the football through the goal post. The goal post consists of two vertical "uprights" and one horizontal "cross bar". Below is an image of a goal post, the ball must pass through in between the uprights and above the crossbar.
An extra point is from a set distance and follows a touchdown. In college the line of scrimmage is the three yard line, a 20 yard kick and in the NFL the line of scrimmage is the 15, a 33 yard kick. A field goal is attempted from anywhere on the field, and is the result of the offense not getting into the endzone. A kicker is rewarded three points for kicking a field goal and one for an extra point.
Another duty of the kicker is the kickoff. A kickoff occurs after every touchdown and field goal. On a kickoff the ball is placed on a tee on the 35 yard line. The kicker then kicks the ball downfield changing the possession to the other team. There are four potential outcomes on a kickoff: a touchback, a fair catch, a return, and illegal procedure. A touchback is when the ball goes all the way through the opposing end zone without anyone touching it. A fair catch happens with the waving of the arm by the kickoff returner protecting themselves from being hit and resulting in a dead ball at the spot of the catch. If neither of those two events happen, and the kickoff returner catches the ball, it is called a return and they are able to try and run back the kick. An illegal procedure occurs when the kick off goes out of bounds on either sideline.
On rare occasion the kicker will attempt what is called an onside kick during the kickoff. It involves kicking the ball and having a teammate recover the football before a member of the opposing team does. An onside kick is a good way for a team to try and come back while losing, by getting possession of the ball twice in a row.
Kickers are only on the field for a handful of plays per game, yet their time oftens comes with the game on the line and lots of pressure on them to succeed.