A convention, or con, is an event held at large scale properties and establishments to gather people with common interests. In the context of chans and the internet, they usually mean anime conventions and/or tech/internet conventions.
Depending on your situation, and your experience, conventions are kickass places to make friends, not feel forever alone, and party all night pissass drunk, or are places where the worst representatives of your hobby tend to congregate at and act like complete fucking dumbasses. If you are a person that feels undignified if people get excited and give hugs to another in humane fashion, then you are a normalfag/cocksucker with no soul, and need to get the fuck out. Furthermore, if you are that kind of person that feels more dignified and higher than everyone else you see around you, you are not better, and are in fact, an elitist faggot; you are not welcome to these places outside of your internet.
Due to how chans are related to Japanese culture, and so is anime, in Japanese popular cultural nomenclature, conventions usually means these:
These places are either kick fucking ass, or you are an arrogant anti social pussyfoot faggot with no skills to even say a simple "Hi!". Depending on where you go, it is exciting as hell or are boring as fuck because of one simple factor: The staff behind it all, or you, essentially. There may also be loads of drama waiting if you are an asshole and/or douchebag, so behave or fuck off (and rightfully so).
Because of the dynamics of anime distribution and creation trickling down onto everything else that tie to it as a relevant subject, these are more social and fandom based, with people gathering for the community and getting to know more through people, with some exceptions, being the larger scale conventions. If you are here, interact with people and do stuff, or you will waste your weekend. Wandering is not uncommon here.
Beware of top notch holy shit weeaboos, ranging from teenyboppers who are out of their minds (increasingly rare these days) to shithead smug ghosting (sneaking, nonpaying) assholes who are complete losers outside of their forums where they reign king.
Remember to BATHE. No one gives a fuck if you will come in for a day, but gods man, everyone will if you don't fucking bathe. If the place is open air, your ass is lucky, but good luck meeting anyone a foot close to you either way.
A lot of people here usually cosplay.
In reality, no one really gives a fuck about furries unless you are willing to cause drama over it, furry or not furry. Either way, either preference, you are getting kicked out if you start drama. Contrary to rumor, 95% of anime convention goers are not furries.
If you cosplay as Anon, NO WORDS. It's better that way. Make sure no one knows who the hell you are. If you talk about 4chan openly, if you brag about Anon, if you gloat about your trolling exploits, if you host a panel on it, if you talk like a /b/ board, then you are, officially, a grade A+ faggot. No one likes /b/tards or chan goers outside of their natural habitat.
No one likes oldfags, no one likes normalfags, and no one likes drama. Also, no one likes people who talk like complete faggots fresh out off of the internet who can't speak what's on their mind and can only speak it through an imageboard. Learn to reality or stay home.
Otakon is a well known convention in the USA, being held in Baltimore, Maryland. The name of the convention is made up of the words otaku and convention, and is the longest running of any anime convention in the US. The convention was primarily focused on fanatics of the Japanese anime subculture, though it has expanded to selling mainstream American media.
Comic Book Conventions
The sort of convention that is, pitifully mocked moreso by mainstream masses, Comic Book Conventions are hit or miss places to have fun at. As these conventions attract /co/ goers and fans of comic books, expect good people or douchebags.
If you meet anyone that cosplays for anime, ask them what the fuck are they doing there and tell them to go to an anime convention (politely).
The mass majority of Comic Book Conventions are usually more trade show like than a relaxed community based one, meaning they are set at huge warehouse like venues with vendors and hosts displaying their wares for sale. Though these can be fun, these are more high strung and with people running around than sharing interests and establishing common ground.
While anime conventions host guests of honor and celebrities known in the Anime industry, comic book conventions usually summon actors, big wigs, and obscure but well respected figures in US geek scenes. Beware if you want autographs from them; they usually charge up to $80 for a picture.
Going to huge ones, like Comic Con, are extreme media hotbeds of activity, usually because current Hollywood actors and figures in the entertainment business come to show off and display news for their fans. It is debatable, but often goers complain that conventions like these are getting cheaper in quality and less connected as a community due to how impersonal and distant people act as years go on due to cherishing meeting stardom over knowing people as a whole.
Because of this, such activity attracts a lot of people who aren't comic book fans. Usually, these are normalfags that clog panel lines for hours on end in the attempt to see their idol/star live in person. Unless you are a die hard for this sort of stuff and are willing to burn your time to see them for a second, it is not worth it, as they usually post the panel online after they record it.
Again, BATHE AND BEHAVE