Eventbrite is an events management service. If you're having an event and want to manage the entire thing online, Eventbrite lets you set up a customized destination page where attendees can register and buy tickets. It can also take the event you created and send it out to other events services in order to promote it.
Eventbrite makes its money off of fees. The more expensive your event is, the more it charges. It also takes care of things like credit card processing, so the people buying your tickets can make the purchase as if they were buying something online.