A swarm is a random match up of area('s) Pokemon such as a bunch of oddish. They're also amazing to fight vs Effort Values (ev's). Anyway swarms are mostly desgined for the player to fight ev's other than that youd never guess it would anyone really.
Heres a short demonstration and a video.. Click the link below
Oddish - ev yield 1x special attack oer monster ko'd.
Fighting a swarm of oddish (5x) will yeild 5x several sp.atk evs and this'll also contribute towards other Pokemon in your party for instance
You use a vileppume to use sweet scent in a particular route when its not raining or snowing if it ever does, lol then sweet scent will call a swarm if your in grass and at an area where there is swarm. Then you get the leader to use a move that'll ko everything like surf or whatever use the leader as something that is useless but powerful.. Cause were not bothered to much about his ev's right.. The rest of the party however will have ev's built up which is good.. Works with other swarms
click here for a list of swarms on Pokemon Omega / Alpha .
Youtube Ev - swarm video 5.32mins only..