Every class can be applied not only to the individual, but also to a squadron. Also, only a character of the given class can form the squadron of that class. Starting Ship Selections means just that, those are the ships you can select from at the start. After that, any class can obtain any ship later in the game. This class is pretty self explainatory.
Pirates make they're way through life by preying on the welfare of others.
A pirate class character would be at least semi-actively involved in raiding choice targets. Whether they be merchant fleets, space stations or even industrial complexs on small moons. Some Pirate's are worse then others, as all Pirate's tend to develop a choice of targets. Some will merely raid, attempting to avoid unessacary casualties or target primarily merchant fleets that aren't carrying vital imports such as agricultural goods. One major drawback is the limitations on starting locations, since not many areas are tolerent of a pirate once discovered.