Off spin is bowled by a right-handed bowler, holding the ball between index, middle and ring finger, with the wrist rotated (rapidly) clockwise at the point of delivery. Spin is imparted by the fingers, which is why this type of delivery is known as 'finger spin'. I've also heard finger spin (bowled from either hand) described as orthodox, which I assume is because finger spin was the first type of spin bowling discovered.
Off spin causes the ball (from the bowler's perspective) to spin from left to right upon bouncing (pitching). From a right-handed batsman's perspective, the ball spins from the side of the wicket his bat is held towards (the offside) towards his legs. In cricket parlance, off spin (to a right-handed batsman) spins from off to leg.
It tends to be easier for a batsman to play a ball that is spinning in to him rather than away from him. I guess this is because it is easier to play with the bat close to the body than play away from the body, upsetting your balance. For whatever reason, playing off spin is considered easier (at least if you bat right-handed, as the majority of players do), and therefore off spin is somewhat dull and unfancied. There have been few really successful offspinners in the last 50 years. True, the most successful spinner of all time, Muttiah Muralitharan, who only retired last year, was an offspinner. But Murali possessed extraordinary physical attributes (something to do with hyper-flexible joints) that enabled him to impart tremendous spin in both directions. Orthodox he was not. Your averagely flexible offspinner hasn't had much of a chance. That said, there have been a few very successful post-war off-spinners, of whom Lance Gibbs, Hugh Tayfield and Saqlain Mushtaq spring to mind.
Saqlain is of critical importance, as he is credited with inventing the doosra. Before Saqlain it was widely reckoned that an off-spinner could make the ball turn from off to leg, or make it go straight (the 'arm ball') and through subtle variations in flight, spin and pace vary the trajectory of the ball such that the batsman never quite knew what it would do, but is was impossible for an off spinner to make the ball spin from leg to off. Then Saqlain started bowling the doosra (literally 'the other one') and the world of the offspinner changed for good.
The doosra is controversial because a) no Englishman (or Australian) has yet managed to bowl it properly and b) many believe it is not possible to deliver a doosra without 'chucking', that is bowling with a bent elbow that straightens as you release the ball. Chucking is big no-no, but is hard to spot when a chap is bowling 50mph plus.
Let us put the doosra controversy to one side, and anchor ourselves firmly in the present day. For, after what seems like forever*, England have unearthed an off spin bowler of genuine, match-winning, Aussie-snaring, talent. Graeme Swann. Enough words, here he is delivering the classic off spin ball to dismiss one of the greatest batsmen of modern times. You won't see a better example of off spin than that.
Tomorrow - More finger spin! Left-handed!!
* whereas it has only been half a century, since the great, perhaps greatest off-spinner of all time, the only man to take 19 wickets in a single Test match, played his last Test in 1959