Finally, after adding all of your guests, it's time to click the "save event" or "create event" button, depending on which page you're on, and invitations will start going out through Facebook and e-mail. Afterward, a new page will come up showing your event with both an "edit event" and "message guests" button in the upper right, which makes it easy to send updates to everyone -- or even just selected attendees. This can come in especially handy for surprise parties, for example.
As invitees start responding, their RSVP information will show up on the event page, which looks similar to a Facebook wall. Each new responder has the option to add a link, photo or video, too, so the event page itself can be a guestbook of sorts for those attending or sending regrets, and a place for all invited to interact.
You can even return to an event page after it occurs by selecting "events" from the menu on the left of your Facebook home page and clicking on the link to "past events" at the bottom. From there you have the option to message guests, send thank you notes or start a thread for others to upload photos or video for posterity.