Twitter is a "microblogging" service, where users post 140-character "tweets". They can be mundane reporting on their life, a wry one-liner, or the start of an Internet campaign that will snowball and end up with international media coverage. Really, for just 140 characters, there's lots of potential.
The site's open, public API allows for its adaptation and use on many different platforms. Android, iPhone, browser extension, desktop software, mobile phone... you can tweet from just about every device going. Hell, you can even monitor tweets with a typewriter!
The site is popular and trendy. The real-time search function, which allows you to search all (public) tweets being made for any word or phrase you wish, is one of the most popular aspects of the site. This is combined with the "trending topics", a list of the ten most popular topics at the moment, based on how much they're being tweeted about. This list of trending topics will almost always include Justin Bieber, at least until Twitter supposedly banned his name from trending and his fans made "let Bieber trend" a trend instead. When they were no longer able to do that, they resorted to trends like "Bustin Jieber". Still, Justin Bieber related topics trend almost every day, and in 2012, they were joined by topics about British-Irish boy band One Direction.
Twitter has gotten a lot of media attention. Major events tend to be covered rapidly through the system, and it has become hip in Hollywood for a celebrity to have a Twitter account. Twitter nicknames get given out like email addresses. In fact, despite the Biebermania, Twitter appears to be the first Friending Network type site (aside from work-oriented LinkedIn) in which thirtysomething media professionals outnumber teenagers.
- Beige Prose: Though it's still impressive how much you can say within 140 characters.
- Character Blog - Many fictional characters have a presence as well. Most are roleplayed (look for the telltale "[RP]" or "role-played" somewhere in their profile), but some are 'genuine' in the sense that their origin company controlled them. You can see a list of some of them here.
- Colbert Bump: A big part of why Twitter is so popular has to do with celebrities having taken to it so enthusiastically. And unlike most trendy things celebrities do, there's a (semi-)practical benefit to ordinary people following suit: People can now address their tweets directly to them and, supposedly, even get their attention this way.
- Fan Girl: Many. Why do you think Justin Bieber never stops trending?
- Food Porn: Look at this awesome sandwich I just made! @everyone
- Game Breaking Bug: As of August 31st, 2010, this has happened to any Twitter application only using Basic Authentication.
- Additionally, the new OAuth implementation is broken itself. It basically requires the identification keys to be hard-coded into the clients, and Twitter has announced they are going to be de-authenticating compromised keys. And, yeah, the keys to the official Twitter client for Android are a string dump away.
- As if they weren't already suffering from an overly severe buttmonkeydom, Zune users had to wait 2 weeks for their official app to be fixed after the switchover.
- I Am Spartacus: After Paul Chambers lost his appeal against his conviction for tweeting a joke about blowing up Robin Hood airport, people began retweeting the original message en masse with this attached as a hashtag.
- New Media Are Evil: Subverted to hell and back by old media's reaction to Twitter. They are crazy for it, and pretty much every old-media organization had set up multiple Twitter accounts (plus dozens of individual personal accounts for employees) before it even really caught on. See also Small Reference Pools, below.
- But played straight by younger media -- for example, read the hilariously hypocritical message board rants against it. Boo, any interpersonal e-communication that isn't e-mail... wait...
- Orwellian Editor: The ability to delete tweets allows a user to be one.
- Please Select New City Hashtag: Sometimes hashtags collide. For instance, #btv had been used primarily for discussions pertaining to Burlington, Vermont (from the city's airport code), until a massive influx of Arabic-language posts in early 2011 referring to Bahrain Television, after which the Burlington folks switched to #bvt.
- Small Reference Pools: The BBC, in particular, is obsessed to an indescribable degree with Twitter. Any news that can possibly be related to anything to do with technology or society in general, never mind the Internet, is determinedly dragged around by its news interviewers to the subject of Twitter, often leading to the more net-savvy interviewees becoming bewildered.
- The Tetris Effect: The practice of "@-replies" has spread to other blog comments and forums.
- From Cheezburger Network's "Failbooking";
OP: It seems like Twitter-style hashtags have replaced HTML-style coding as Internet shorthand for meta-commentary.
- Unperson: Twitter's "recalibration" of their Trending Topics algorithm was obviously meant to do this to Justin Bieber.