What is the difference between a chinaman bowler and a leg-spinner?
The trajectories and direction of both the deliveries are the same.
The trajectories and direction of both deliveries are not same.
A leg spinner bowls right-arm with a wrist spin action, causing the ball to spin from right to left in the cricket pitch, at the point of delivery.
While Left-arm unorthodox spin bowlers (i.e. chinaman) use a wrist hand action to spin the ball which turns from off to leg side (i.e. left to right) of the cricket pitch.
Chinaman's delivery vs Leg spinner's delivery
And why do they call it Left arm unorthodox spin? I can understand unorthodox but why just Left hand?
That is because very rare bowlers bowls this type of delivery. And this type of delivery first seen in 1933 which was bowled by Elliss "Puss" Achong who was a left-arm orthodox spinner. Since the ball he bowled was not orthodox for a left-arm spinner, it was called as unorthodox. The story behind why such bowlers are called as chinaman is given in Wikipedia (emphasis mine).
The name has its origins in a Test match played between the West Indies and England at Old Trafford, Manchester, in the year 1933. Elliss "Puss" Achong, a player of Chinese origin, was a left-arm orthodox spinner, playing for the West Indies at the time. According to folklore, Achong is said to have had Walter Robins stumped off a surprise delivery that spun into the right-hander from outside the off stump. As he walked back to the pavilion, Robins said to the umpire, "fancy being done by a bloody Chinaman!", leading to the popularity of the term in England, and subsequently, in the rest of the world.
We all know Brad Hogg as a leg spinner. But Wikipedia states him as a Chinaman Bowler.
No. Brad Hogg is not a leg spinner. He is a chinaman(or Left are unorthodox spin) bowler as I have already described.