I think this is how it works.
When we see sasuke first, he has one spiral in his sharingan.
Kakashi has two spirals in his sharingan.
Itachi has three spirals.

My guess is that:
1 spiral = ability to see through high speed movements, genjutsu and ninjutsu.
2 spirals = ability to copy ninjutsu and memorize taijutsu movements.
3 spirals = ability to enter someone's mind by looking at them, bascially master genjustu. This is what happened when Itachi tortured Kakashi just by looking at him. However, I don't think it can be used to control minds.