A form of expressing laughter when oral expression is not available, like on the internet.
Hah - The person thinks a comment is mildly funny.
Haha - The person does not think the comment is funny, but acknowledges your attempt humor.
Hahaha - the person thinks the comment is funnier than average, or is just more enthusiastic than those who normally say haha.
Hahahaha - The person is just being excessive. Usually used to strengthen a friendship because an inside joke was just mentioned.
Haha is almost never used to express actual laughter occuring. For those circumstances, rofl, lmao, or a whole sentence about how the comment actually made one laugh is used.