It may seem odd to ask what it means to laugh, but there are different qualities of laughter. After all, is it not different to laugh at a joke than to laugh at someone's misfortune? Also, not everone is able to laugh at themselves. Or at a bad personal situation.
Some see humor as I do, an extremely important quality. Others see it as something that gets in the way of getting things done. Most are apatheic to its qualities, though they still enjoy a good joke.
By itself, I see humor as being able to see anything as funny. Like I mentioned before, this is not always good. There is little useful humor in someone falling down a flight of stairs. Sometimes, though, we can look back at a personal incident in the past, one that was not funny at the time, and laugh.
I think I'll leave this post short and go with a general definition of Laugh and humor in general. Feel free to contest this in the comments.