Pirates refers to various things in UO.
Sometimes, in the days before Trammel, certain Reds would wander the land on boats, attacking other players on boats, and they were referred to as pirates. Some would Roleplay (see Roleplaying the part, and others would not.
Various EM Events on certain shards featured NPC Brigands called pirates. They were also used as part of the Vanguard event in some of the cities. However, in the latter appearance, they were bugged, and could neither damage players nor be damaged directly by players (pets would work under certain circumstances).
With the release of the High Seas booster, Pirates are started to be seen around Jhelom who are the captain of Orc Ships with Orc crew members. The pirates are captured or killed to complete the Professional Bountyhunter Quest. The names of these pirates are randomly selected between developers, game masters and veteran players.