PUG: Pick-up group. A group of indibiduals who sit in a queue for a mission and don't form a team until they get into a mission. This leads to a lack of coordination, compared to pre-arranged teams, and you're less likely to win the mission or get the optional objectives.
STF: Special task force. A five-person mission that cannot be played solo, and requires more skill and coordination than regular missions. (At least, that's the aim.)