They are a criminal gang. Biggest one around. They-- well, they get involved in a lot of different crimes, but their main thing, or at least the main thing your average person is going to have to deal with, is that they steal Pokemon from folks. Usually, pokemon that are valuable or unique in some way, so either ones that are strong, hard to come by, or shiny.

Don't count on the police here to deal with them either. They are useless.