The classic definition of "semi formal" is suit and tie for guys. Nice dresses for ladies.
The practical definition of semi-formal can be that or less, depending on the context. The "semi-formal" dance at my dorm turned out a few people with suits/nice dresses, a few people with shirt and tie, and a few people who wandered down dressed in normal clothing. I've been to semi-formals where most people dressed according to the classic definition, though. If you enjoy dressing up in a suit and tie, go for it. If you don't want to feel overdressed, hedge your bets by wearing a shirt and tie but no jacket.
Geez, I feel like a male Miss Manners. 
 
 
BTW, you have another copy of this thread on the board. Might want to edit that to a "." in the title or something.