Thrust reverser: There are a number of types, ranging from bucket reversers that swing a pair of doors into the exhaust stream, directing it forward, to door-based systems that direct the bypass air of a high-bypass turbofan engine out the sides of the engine in a somewhat forward direction.
In most cases, you want to apply reverse thrust as soon as the wheels are on the ground. Some aircraft can use in-flight thrust reversal for a quick reduction of speed or altitude, but these tend to be military aircraft that need the improved performance. You want to stop using reverse thrust as soon as you're going slowly enough that any debris kicked up could make its way into the engine intake, which is why airplanes mostly don't use reverse thrust for backing up.
As a special case, turboprop engines can twist the propeller blades so that the normal rotation of the propeller pushes air forwards rather than back, but you may not consider these to be "jet" engines.
