Pins
A pinned piece cannot move without exposing something behind it. Learn to pin, and to win the pinned piece.
What a pin is
White's bishop on b5 attacks the knight on c6, and Black's king stands behind it on the same diagonal. If the knight moved, the king would be in check, so it may not move at all. That is an absolute pin. When the piece behind is not the king (a queen or a rook, say), moving is legal but costly: a relative pin.
Pin and win
Your turn
Black's queen and king share a diagonal. Pin the queen and win it.
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Bc4 pins the queen to the king. She can only take the bishop, and the b-pawn takes her back: a queen for a bishop.
Absolute or relative
Question
A white bishop on g5 attacks Black's knight on f6, and Black's queen stands behind it on d8. What kind of pin is this?
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A relative pin
The knight may legally move, but then the bishop takes the queen. Moving is allowed, just expensive.
More in Tactics
The knight fork
One knight, two targets. Learn to spot squares where a knight attacks the king and a valuable piece at once.
Skewers
Attack a valuable piece so that when it steps aside, the piece behind it falls. A pin turned inside out.
Discovered attacks
Move one piece out of the way and unleash the one behind it. When the hidden piece gives check, it is a discovered check.