A sweeper is a Pokemon that usually has high speed and either high attack or special attack (sometimes both). Such Pokemon include Archeops, Gengar, Latios or Jolteon.
Most teams tend to have around 2 sweepers. A sweeper's job is to either knock out a few Pokemon that do the rest of your team harm, or to clean up the rest of an opponents team after they have been significantly weakened, hence why it's called a 'sweep'. Since sweeper's rely heavily on a single stat (most of the time), taking out very specific counters, as well as general walls, is vital, especially since most wins are a result of a late-game sweep (though clean sweeps from the start can happen too). This playing to take out specific components of a team to then be able to beat the rest of it is how competitive battling works, on a basic scale.
Sweepers can be branched off into new categories too, such as Glass Cannons, Bulky Sweepers or Revenge Killers. This are a little more specific and require different qualities to work than a generalized sweeper, so are more complicated.
Accidental Torkoal sweep in OU against all odds - Blob
Replay taken from our Archive.