Reapers are advanced Terran infantry equipped with dual pistols and a jetpack, intended as fast-attack and harassment units.
In Starcraft, Reapers are fast-moving, jetpack-equipped medium infantry armed with high-powered pistols and explosive charges. Predominantly they are used in Starcraft to harass and disrupt enemy harvesting operations (often deployed in trios for this purpose), and can inflict considerable damage against light armor. Left to their own devices, they are also dangerously capable of hurling enemies all over the place with their KD8 Charges.
In Starcrafts, Reapers are predominantly attack infantry, sent out usually in a small team when a counterattack is required or when everything else screws up. Like many running gags in Starcrafts (the Immortal's armor and firepower, for example), their raw attack power is exaggerated and they are often portrayed as being able to take out enemy facilities with just a small squad.
Gung-ho, aggressive, and always looking forward for an excuse to blow things up, Reapers in Starcrafts are always looking for an excuse to run riot. Despite the aggressive, often insane personalities they exhibit, Reapers are (ironically) one of the most competent infantry units that the Terrans have; in Alliance, a small squad of them is responsible for obliterating the main red base and leaving it a burning pile of ash, ultimately allowing the Purple Zerg, Blue Terran, and Blue Protoss forces to win the day when all of the red team's structures are destroyed, causing them to lose the match.
Very often, their attacks involve rampaging through the enemy, breaking windows, firing into the air to scare the local worker population, starting fires, and generally smashing up the enemy's structures or gunning down drones and probes.