Gunners are lightning fast and have an ability to pass blockers on the opposing team to reach the returner. Gunners usually run on the sidelines of the field immediately when the ball is kicked to get to the returner as soon as possible. They can begin running as soon as the ball is snapped, trying to evade the other team's blockers.
A jammer is a player on the special teams that counters the gunner. During a return, jammers are on the receiving team and are responsible for stopping gunners from tackling the returner. Sometimes, a team will assign two jammers to a single gunner. This is because the gunner is so quick that it often requires more than one player to stop him. Jammers try to get the ball and run it back down the field for a touchdown.
Gunners are known for "gunning" it down the field, meaning that these players have to be very fast. However, a gunner's course down the field is not usually linear. He has to run a lot of ground in a very short time, so he has to be fast on his feet and agile enough to change directions in an instant. They must also be able to punt and catch the ball well. These situations only last a few moments, but can be the deciding factor and make or break a game.