Patrol craft are small warships designed to act as a security force for a nation's military. They are typically seen performing border patrol duties on planets, carefully inspecting passing ships. They also see themselves acting in rescue missions, as their small size allows them to get in and out of bad situations quickly.
Patrol craft are the smallest "true" warship, even smaller than corvettes. Their small size and oftentimes streamlined builds make them among the fastest ships in the universe. This is especially handy when it comes to chasing down the heavily-modified ships that smugglers and other criminals usually use.
While patrol craft are armed, their weapons are usually too weak to do any meaningful damage to ships above their own weight class. This means that most nations will not field patrol craft on the frontlines, keeping them in reserve in case of emergencies.
However, where patrol craft truly excel is performing search and rescue operations. Their size and speed allow them to enter into hazardous areas, extract the person or people in danger, and rush out better than any other ship.