Weird but interesting question: What is a fart?

Exactly what is it, why does the human body perform this action, and it must be something significant for bodily function, or else the body wouldn't do it as much as it does. Thanks for not saying "gross" and that's it because I've seen that and it's a waste of room for sofisticated answers. Please just explain and be done with it and we will all be happy. Thanks again! :)