On a 9 seated table for example:
Seat 1, button
Seat 2, Small Blind
Seat 3, Big Blind
Seat 4, Under the Gun (UTG)
Seat 5, UTG + 1 (early position
Seat 6, Mid Position
Seat 7, Mid Position
Seat 8, Late Position
Seat 9, Cut Off (C.O.) late position
Seat 4, UTG, is the weakest spot as first to act. Under the gun should only be raising with very strong hands, or calling with good hands like QK hoping not to see a raise.
Seat 1, the button, is the strongest position. Everyone else has acted apart from the forced bet positions (blinds). Therefore you have the opportunity to 1)fold to strong plays, 2) raise multi-way limped pots or 3)raise and steal the blinds if there has been no action before you.
Seats 2 and 3, the blinds, are great seats to trap with monster hands with as the button is highly likely to try and steal un-raised pots without giving much credit to the blind positions. They are also good seats to re-steal from the Button as he, in turn, often wont get credit for a strong hand.
Generally, the later you are to act, the more aggressive you can afford to be. Try to stick to late position raises after 4 or 5 players have folded. Its just a case of narrowing your field.
Recently though, a lot of pro's have been getting tricky with the UTG position as its not a traditional move to raise from there unless you're holding a monster. Its slowly becomming the new 'stealing' position. Dont worry too much about that though, that will come with advanced strategy.