It is an online platform that allows you to create and manage events of any kind (from Weddings to Conferences, Exhibitions to Concerts, etc).
It´s easy to use: just sign up and then begin creating events.
Eventbis will generate a Microsite for your event, which can be used to RSVP to events, collect payments, register gifts or for ticketing. The Microsite can also be used in conjunction with web pages or social network sites.
The best way to better understand it is to give it a try!