Now I am using Canon 5D mark II, I want to upgrade my camera into Mark III. I want to know what is the difference between each them.
That is any easy thing to found out :) Just compare the cameras. Here I've done it for you.
As you can see there are several differences. 22 MP vs 21 MP which is really pretty much the same and so is 0.2" difference in LCD size. What is much more significant:
Here's what I would say are the differences roughly in order of how much they actually matter. If you are predominantly a stills photographer:
If you are predominantly a videographer:
The two main deciding factors are:
61-AF points on mark 3
much better ISO performance in mark 3.
You may only want more AF-points for tracking objects, but I believe that even if you do still photography is good to have the option to avoid focus and recompose. I have a Canon 550D, with 9 AF points, like your mark 2, and sometimes it can frustrating, trying to focus and recompose, because of the lack of an AF-point. I know that the central AF-point is always the fastest and the most accurate, but there are times you can't follow this.
High ISO levels are much more in mark 3 and the noise is much less, according what photo examples on the Internet tell.
Hope it helps somehow!