What is a Pokemon Swarm?
While you explore the island of Sinnoh in Pokemon Diamond, there are regularly groups of Pokemon that band together into a swarm. These swarms occur in various Routes around the island and each day a new type Pokemon swarms. If you travel to the area where a certain type of Pokemon are swarming, you'll easily be able to locate that type of Pokemon. After battling one, you have the option to attempt to capture it for your collection.
Why Bother?
As previously stated, the ultimate goal of a Pokemon trainer is to capture one of each type of Pokemon. In the Diamond version of the game, swarming Pokemon can only be captured during a swarm. You will not run into any of the swarming Pokemon types anywhere else in the game. Ignoring the swarms will hinder you from capturing the swarming Pokemon as well as the stronger types they eventually evolve into.
Where are They?
Each day a new type of Pokemon will swarm in a new area. In order to keep abreast of the situation, you'll need information. Your rival's sister can help you with this. Head to Sandgem Town, which is located in the Southwest corner of Sinnoh. You'll find your rival's sister in their home. Speak with her each day to find out which type of Pokemon are swarming on what route. Travel to the specified location to catch your Pokemon.
How to Catch a Swarming Pokemon
After traveling to the location of a known Pokemon swarm, walk around in a grassy area to attract wild Pokemon. It can take a couple of battles before you are challenged by the Pokemon you're after. Attack the monster and wear away its hitpoints. When its HP is nearly drained, throw a Pokeball to attempt a capture. If the Pokemon is worn out enough, the attempt will be successful and you'll have a new monster for your collection. If not, the Pokemon will escape and you'll have to continue battling and try again.