To stop all the arguments on the court, here is the difference very simply. A let is when your opponent stops you getting to play the ball, and a stroke is when your opponent is in the way of the ball travelling towards the front wall when you are going to hit it.
Harrow School in the 1800s is the birthplace of squash rackets, which then became known as squash.
White ball – Used on a glass exhibition court in competitions.
Double Yellow Dot – Slowest ball and used in competitions
Single Yellow Dot – A bit slower than the Red dot.
Red – Fastest ball, used by beginners.