An off spinner is a term referred to a spinner who bowls off breaks. In other words, an off spinner's delivery spins towards the leg side after pitching (all references made with regards to a right hand batsman).
Apart from the regular off break delivery a reputed off spinner normally possess a variety of deliveries in his/her repertoire. They usually possess a top spinner - the one that goes straight through, a faster one - one that is bowled quicker through the air and a very few top notch off spinners possess the wrong one - the one that spins the other way.
Muttaiah Muralitharan of Sri Lanka and Saqlain Mustaq of Pakistan are some of the off spinners in contemporary cricket who can successfully bowl the wrong one.