Nuns and sisters?? Two names for female monestics. The same thing.
Nope. Nuns are monastics. Their vocation is to live a life of prayer within a cloistered convert and have little or no contact with the outside world.
Sisters are those who belong to religious orders that have ministries out in the world: teaching, missionary work, caring for the sick, etc. In popular parlance they get called "nuns", but ask any Sister and she will quickly explain to you the difference.