Its basically an outburst of Pokemon that you would not normally find in that particular area. This lasts for one day on a random route that is selected and notified by:
Swarms are Pokemon that aren't usually found in that game.
In B&W, you can find swarms by checking the board inside the gates from route to route. You can only get swarms after you have the National Pokedex.
Swarms occur after the main story of the game is beaten and a swarm is when a group of Pokemon that you wouldnt regularly be able to catch goes to a random route for the day, you can then catch them there for the day.
Pokemon that can only be caught at certain times at certain places. Outbreaks are on radom routes at random times. You can see where the outbreaks are at any orute change place (Little rest stop place).
It's when Pokemon usually not found in an area have a mass outbreak there