There are many positions and tactics and ideas that occur in chess. please post the ones not yet listed,  and if possible what it is called. I will start off with the basics.......

     ..................opposition and fork

skewer and pin  

this one has no name, but comes up in alot of games

 

It's a pin.

Many players make the mistake of thinking that a fork is when you attack two pieces - it can easily be a square and a piece:

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 It could be just as easily argued that the knight forks the Rook and the a-pawn.  With that said, I think most people will understand what your trying to get at.

My two:

http://www.chess.com/chessopedia/view/windmill

http://www.chess.com/chessopedia/view/luft

Good point Blackadder, my example was bad. This one shouldbe better!

Indeed!   a much better example.

RR: I'd be more inclined to call that a more general 'double attack' and leave the 'fork' tag for a genuine attack on 2 different bits of material.

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 I meant that exact position where you win the rook using the bishop after pinning the pawn