Definitions: Pinning

The idea is easy, exploiting them apparently gives me more trouble than i should like.

Pinning: A lower value piece is immobilised by being in front of a higher value piece on the same line of attack.

The pinned piece might also be referred to as a 'blocker', see point 3 of 'What can be Done'.

Note also the difference between a Pin and an X-Ray; with an X-Ray the higher value piece is in front, if it moves, a lower value piece behind it is exposed to attack.

Reinfeld Position #1
White to move

Black's Queen is pinned to the King. White exploits this with 1.Rd8 an X-Ray attack, 1. ... Qxc4, 2. Rxd8+ a Zwischenzug.

A Pin is more a positional feature than a tactic in it's own right, since an additional motif is usually required to exploit the pin.