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This area of the rules is really complicated, but let me show you two classic interference situations to help explain the difference between a let and a stroke.
In the first one, one player is denied access to the ball. The opponent is trying to get out of the way, and the next player to play or "the striker", can't get round her opponent.
This is a let ball, and the rally is replayed.
As a rule of thumb, if someone is trying to get to the ball and can't get there because of their opponent is in their path, it results in a let.
In this second situation, the striker can't play the ball, as her opponent is in the way. The difference here is that she is ready to play the shot only she can't swing because she would hit her opponent.
This is a "stroke", and the striker is awarded the rally.
The rule of thumb here is that the striker is at the ball, and still can't play a shot.
It's important to note that this is a really simple way of looking at lets and strokes, but it holds true in most of the situations you'll find yourself.