Left-arm spinner is a term referred to a spinner who bowls with his left arm, with an action similar to that of an off spinner. Although the bowler's action is similar to that of an off spinner his delivery on pitching spins towards the off side (leg breaks), the reason being, he bowls with his left arm (all references made with regards to a right hand batsman).
A left-arm spinner also possess in his repertoire an arm-ball. An arm-ball is a term referred to a delivery, which after pitching instead of spinning away (that is spin towards the off side) from the batsman continues to carry on in its line, hence ends up coming in to the batsman. Now, this is achieved when the bowler uses his arm to have the ball come in to the batsman. Since the bowler brings the ball in by using the line of his arm, where he does not utilise the services of his fingers but along with the arm lets the ball go, the delivery is referred to as an arm-ball.
Apart from the arm-ball they posses a faster-one (one that is bowled flatter and quicker through the air).
Daniel Vettori of New Zealand along with Sunil Joshi of India are some of the reputed left-arm spinners currently ruling the roost in world cricket. Bishen Singh Bedi of India ranks as an all time great when it comes to the art of left-arm bowling.