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.
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.