Science fiction has a long history of fan involvement in the genre. Every weekend in the year there is at least one convetion of science ficiton and fanasy fans somewhere in the US. I live in the Washington, DC area and we have our own local convention called Capclave (from Capital Conclave) that happens every October somewhere in the DC suburbs. This year it starts today, in Gaithersburg, at the Hilton Washington DC North/Gaithersburg.
We also have Balticon, which, not surprisingly, is in Baltimore, although really it's north of Baltimore at the Marriott Hunt Valley Inn, and it's always on Memorial Day Weekend. Further north, if you feel like driving, is Philcon (now in Cherry Hill, NJ) which happens in November, and this year they're celebrating their 75th anniversary. Further south and much newer is RavenCon, in Richmond, VA (or its suburbs), which happens in April.
Cons are fun. Usually one or more writers are invited to be Guests of Honor (GoH); sometimes there are also editor, artist, and fan GoH's. Local writers (and some not so local) and fans attend, with some of them serving on panels in which topics are discussed (e.g., Why do so many adults read YA fiction? What is the appeal of steampunk and will it last as a subgenre? What is the "new weird" and is it still new?). Folks who buy a membership can attend panels, shop in the dealers room (which always has books and often has jewelry, craft items, costumes, clothes, etc.), see the art show (if the con is big enough to have one), and hang out in the con suite where free sodas and nibbles are provided.
There are cons with a wider scope, too, like Worldcon, Dragoncon, and World Fantasy Con (not that big attendance-wise, but that's on purpose). Dragoncon is always in Atlanta but the others move around and can even be out of the US. There's also ComicCon, which started as being just for comic book fans and morphed into a huge media extravaganza with TV and movie stars and crowds so enormous sometimes you can barely move. There are now ComicCons in different cities, but I don't really consider them science fiction cons.
There are movie and TV-based cons as well as literary cons. If you've never been to a con, and you think you might be interested, visit this list to find a con near you!
Breaking news! Between when I started this post and when I finished, it was announced that Sir Terry Pratchett will make a surprise appearance tomorrow at Capclave! Quite a coup for a small local con! If you're in the DC area, be there!