What is a swarm in Pokemon Diamond?

Answer 1

In Pokemon Diamond and Pokemon Pearl, a swarm is when a group of Pokemon appear in a large quantity in a specific place.

Most of the time if not all the time they will be bug type Pokemon. To find these swarms, go to Sandgem Town and walk into the house directly south of the town Pokemon center. The sign outside this house says "Dawn's/Lucas's House" (depending if you're a boy or girl). The girl inside this house will tell you the type of Pokemon that is swarming and where they are. Both the type and location change from day to day.

These swarms only appear after you've received the National Pokedex.

A good method of finding Swarm Pokemon is to use a Repel and look for a moving patch of grass which is brighter than the rest. This contains a Swarm Pokemon.