In addition to the excellent information in Bellatrix's answer, I want to add a few more details that we know: (all non-book quotes are from the Chamber of Secrets article on Pottermore)
Salazar Slytherin had a grandiose opinion about his importance to the school:
However, it is clear by the very decoration of the Chamber that by the time Slytherin finished it he had developed grandiose ideas of his own importance to the school. No other founder left behind them a gigantic statue of themselves or draped the school in emblems of their own personal powers (the snakes carved around the Chamber of Secrets being a reference to Slytherin’s powers as a Parselmouth)
In addition to the question of taking in Muggle-Borns, he also disagreed about whether to teach the Dark Arts.
Perhaps, when he first constructed the Chamber, Slytherin wanted no more than a place in which to instruct his students in spells of which the other three founders may have disapproved (disagreements sprung up early around the teaching of the Dark Arts).
While Professer Binns and the Sorting Hat merely say he left or departed, seemingly of his own accord -
After a while, there was a serious argument on the subject between Slytherin and Gryffindor, and Slytherin left the school.” (COS)
And at last there came a morning When old Slytherin departed (OOTP)
Later canon seems to say he was forced out.
What is certain is that by the time Slytherin was forced out of the school by the other three founders,
Also, while Professor Binns says that the fight was between Slytherin and Gryffindor, and the Chamber of Secrets article says the fight was between Slytherin and the remaining three founders, the Sorting Hat says they all fought with each other (even Hufflepuff and Ravenclaw fought with each other):
“And never did they dream that they
Might some day be divided,
For were there such friends anywhere
As Slytherin and Gryffindor?
Unless it was the second pair
Of Hufflepuff and Ravenclaw?
So how could it have gone so wrong?
How could such friendships fail?
Why, I was there and so can tell
The whole sad, sorry tale."
Considering that the Sorting Hat was there at the time, it seems likely that his version is correct, and the fight was between all the founders, and that Slytherin may have actually left of his own accord.
Furthermore, the Sorting Hat (deliberately, based on the aftermath of the song) seemed to imply that all the founders were equally at fault for the fights, unlike Professor Binn's version, and the one at Pottermore, which imply that Slytherin bears the main brunt of the blame.