why would you trade a pawn in for anything other than a queen?

i mean i'm new to chess but the only thing i can think of is maybe a knight in a rare exception but how often does that happen??

Bishop Rook Pawn KING!!!

what i mean to say is when the pawn gets promoted

A knight or a rook to avoid stalemate.  I've seen one puzzle where a bishop was required, because a queen stalemated and the bishop gave the fastest mate, but in real life a rook worked fine.  Also its kinda fun to be in a OTB tourny and have a TD director come over and be really confused about why you have two dark squared bishops.

Look at this position from an actual game. You would promote to a bishop to win the game.

pawn =1
Knight = 3
Bishop = 3
Rook = 5
Queen = 9

The values of the pieces depend on position. A passed pawn can be worth more than a bishop or knight, a bishop pair is worth more than a knight pair, etc.
In general a pawn is only 1

edit: now i understand, you mean promoting.
In 98% I promote to queen, on occasion to a knight or rook (if it doesnt matter, rooks are more predictable and less likely to stalemate) though that was when I was beginning to learn

to prove you're a stud

oh ok.... it's still a little over my head at this point but i'll take your word for it.... thanks!!

I have seen games in which the pawn was promoted to a Knight cuz that resulted in a checkmate whereas a Queen would not have been able to reach the same square.  However, I honestly don't understand how promoting to a Rook or Bishop helps in any way whatsoever, cuz there's nothing they can do that a Queen can't.  How does promoting to a Rook or Bishop instead of a Queen avoid a stalemate?  Forgive my ignorance.

This is an interesting topic- and actually have seen a game played by the French prodigy Maxime Vachier-Lagrave in which he promoted a pawn to knight for a beautiful mating combination- whether this was actually necessary, I didn't really check, but as rganesh said, he certainly proved he's a stud.

Thanks for asking this question, I wondered why you would ever want a bishop or rook. 

lostaccident, in the game above, pawn takes rook and promotes to bishop, because black has only one legal move, with HIS bishop, after that,  and since he could NOT move it if you had a rook or queen, as one cannot move INTO check, promoting to a bishop (or, of course, but unnecessarily, a knight) is the only move possible to avoid a stalemate, assuming that you would take his rook on your move.

There may be scenarios where choosing a queen could put you into stalemate, where choosing a rook would otherwise allow you to win the game.

Personally i've never encountered a situation where I've chosen anything other than a queen, though I have seen games where a knight promotion will win quite elegantly (as a previous poster mentioned).

ahh... i see it now AdrianMonk -- that's for spelling it out for me.

The pawn promotes by capturing the rook, not by going straight forward.  So, the promoted piece is next to the black bishop, and its now black's turn to move.  The black pawns are stuck.  Black's king can't move to the white square because its controlled by white's bishop, and can't move to the dark square because its controlled by the white pawn. 

If white promoted to a queen, then the black bishop would also be stuck.  Without a legal move, black is in stalemate and the game ties.

If white promoted to a bishop, then black has a legal move, which is to move the bishop somewhere, giving white the opportunity to continue playing to a win.

There's another possibility where promoting to a knight instead of a queen gains material. Imagine K, Q, and P vs K and 2 P, one of which is about to promote. Promoting to Queen would just leave forces even; but suppose the promotion to knight forks king and queen, leaving a piece up and a won endgame.

 Same exact outcome if you promoted to queen instead of bishop

Let me remind you something:

http://www.chess.com/forum/view/more-puzzles/a-puzzle---one-of-the-finest

http://www.chessvideos.tv  endgame study(saavedra position)