This is not what to use twitter for. That may come later, thinking about it.
You’ll notice people putting things like #stuffmydogate in their messages. These are hashtags, and they’re not special, they just provide a handy and unique thing to search for to find more things along the same theme. So all the messages about the trial where someone got lawyered for a terrorism joke get tagged #twitterjoketrial. Handily, Twitter itself – and most clients – will link such words to a realtime twitter search for the use of that tag. They’re not registered anywhere, but their nature tends to mean they become trending topics if they take off.
Originally, Twitter didn’t support any reply mechanism, but people started prefixing their messages with @username if they wanted to direct them. Twitter noticed this, and provided highlighting for this behavior, and eventually changed the system so that if someone else on your list starts a message with an @username to someone you don’t follow, you never see it.
Clients and the web interface offer you a “reply” button, which appears to just prefix a new message with “@username: “, this is actually hiding stuff as the “reply” function sends a hidden field identifying the message you’re replying to, which in turn allows the interface to show “this message is a reply to that specific message”. Generally, if you’re replying to a person, just create a new message and start with @person. If you’re replying to a message, use the reply button on your client.
Maximum username size on twitter is 15 (a few people have ones longer than this due to bugs that have cropped up over the years, but you shouldn’t be able to create them). If you’ve ever wondered why Twitter is 140 characters, it’s because the original system was designed to work over SMS as a primary medium. To send a direct message, you had to write “DM USERNAME your message goes here”. Messages are 140 messages to always comfortably fit into this format.
Twitter lists are groups of twitter users maintained by a specific user. You can subscribe to one of these lists in various clients and when you view them (in a column in Tweetdeck, for example) you switch to a kind of alternate timeline where you only see these people.
Following prolific people
There are various people on twitter who talk an awful lot of crap an awful lot of the time. Stephen Fry is an awesome human being and Kevin Smith is tremendously entertaining, but once they get their text on they can quite comfortably drown out everyone else you follow. My advice here is not to. “Follow” is a specific action, it means they appear in your main timeline. Personally, I try to keep my main timeline clear so I have a snowball’s chance in a supernova of keeping up with it (I try, and I fail. I directly follow 339 people right now). Prolific people and people who I want to read but don’t care that much about get relegated to lists as defined above. This way I can happily turn on notifications for my main timeline, and check the celebrity list every so often.
This approach works very well with the various fictional groups on Twitter, too. For example, the main characters of Questionable Content have their own twitter accounts, and reading them in the list makes a lot more narrative sense than mixed in with #whatmydogate from the rest of the twitterarti.