The Stroke: An in-depth look - Squash Rules -

The Stroke

Whenever there is an impediment to you making a winning shot, caused by the fact that your opponent is in front of you or inside of your racket swing space. A point will be assigned to you. This judgement is called stroke.

A winning shot is a shot that goes directly into the front wall.

Therefore everytime your opponent throws a shot near himself, and you are behind him, you will receive a point and a stroke will be assigned against your opponent.

When does a stroke gets assigned

A stroke can be assigned for many reasons such as:

1. If you hit your opponent.
2. if the conditions for a stroke are applicable after the ball bounces away from the back wall provided that you don't rotate yourself (I.e. you keep facing your opponent, this typically happens if your opponent makes a really terrible shot). 
3. If your opponent is inside of your swing when you are making the shot.
4. If you hit him with the racket after making the shot, the stroke will be assigned provided that you haven't done it on purpose (I.e. you made a regular, 180 degree maximum swing.)