awknowledgement of someones comment or joke, but indicating it wasn't funny.

what lil wayne always says in the middle of his songs

Like lol, it is meant to convey a laugh in texting and instant messaging, but unlike lol, you must actually have laughed before typing it. It also implies more confident laughter.

Basically it's a way to say lol, without having to say lol. It's great.

some times used to show amusement.
sometimes used to show that what the person is saying, doing, trying to do,or asking is complete unresonable on your standards.

When someone tells you a joke/story that's not funny.

When you say or type "ha" the general understanding is that you "had a little laugh", because the joke wasn't any good or interesting, but the actual thing that is in your mind when saying or typing it is "how annoying..." ("ha" is used as acronym)

the most ghetto-est way to say "her."