Crew skills in SWTOR are the same as crafting, tradeskills and professions in other Mmorpgs. The big difference though is that you are not actually doing the crafting and usually not the gathering. You send your companions out to do these things for you, while you cover the costs. These costs are usually covered by the money you earn from selling the items later. Having companions doing these things for you makes it possible for you to send a companion out to craft or gather while you spend your time leveling, pvping or whatever you want. Try to always have your companions out doing missions when you don’t need them.