See also: Arisia, Inc.
If you've been to a science fiction convention, the answer is easy. It's people, panels, movies, dealers, an art show, a masquerade, parties, late night conversations in hallways,and a chance for science fiction fans and professionals to congregate under one roof.
If Arisia is going to be your first science fiction convention, then you're in for a treat. Bring (or meet) friends if you can. If not, don't worry. This is a gathering of fandom. You'll have ample opportunities to meet people. If you're shy or don't easily operate where everyone seems to know everyone else, then volunteer some time to work the convention. That will give you an automatic excuse to talk to people, including some of the biggest names in science fiction. (As an added bonus volunteers are fed, given t-shirts, and can even earn a free membership to next year's convention.)
The important thing to remember is that unlike a commercial convention, Arisia is all fan run. No one is being paid, including the professional writers and artists. People are involved for the same reason you're interested: they love science fiction.
There's so much to do that you'll never do it all, so just pick and choose and remember there'll be another Arisia next year - and the year after that. You'll find yourself making friends that you'll look forward to seeing at the next Arisia, and perhaps other conventions as well.
(Some fans are known to socialize in real life, too. There have even been cases of fans getting married, creating little fans, and bringing them to Arisia. Our lawyers insist we can take no responsibility in that regard, but we will offer you our congratulations.)
If you're planning on coming to your first Arisia, welcome. You're going to have a great time.
Thanks to Dan Kimmel
Many SF conventions are called something-or-other-"con" (e.g., Lunacon). Way back in the 1940s, a Boston SF con was envisioned. The obvious name was "Boscon", but it was decided to engage in a bit of wordplay and call it "Boskone" instead, after an organization in the classic "Lensman" novels by E. E. "Doc" Smith.
Many years passed. In the late 80s a group of fans decided to form a new convention, separate from Boskone, to serve the New England area. The name "Arisia" was suggested, that being the name of a planet from the "Lensman" books. Et voila.
It is usually pronounced like "anesthesia"; "ah-REE-zha". So long as we know what you mean, we rarely feel like debating the issue.