Twitter has received a lot of attention lately, but some people still do not know what it is or why it is so popular. Twitter is a microblogging platform, which means short text updates on what you are doing. This is similar to the What's on your mind box on Facebook, but with far more flexibility. Messages are almost all public. That is, anyone can read messages from/to anyone else, unless they are explicitly marked as hidden. This may seem a bit strange, but it makes communicating and catching up on conversations much easier.
A message is sent to an individual by prefixing the message with an @ and their username. E.g.: "@acperkins I like your T-shirt" would flag up in my timeline as a message to me. For a private message that no-one else would see, you can use the Send xyz a direct message feature or you start your message with a 'd' and the username. E.g.: "d acperkins Where are you?" would appear in my Direct Messages, but no-one else would see it.
Twitter is easy on your privacy. When you sign up you do not need to supply your birthday, town, or anything except an email address. If you supply a mobile phone number then you can text your status, and if you are on Vodafone then updates will be sent as text messages to your phone.
The greatest advantage of Twitter is the huge number of apps to interact with it. There are apps for mobile phones, PCs, Macs, PDAs, blogs, toolbars, sidebars... the list goes on. These apps make it simpler and easier to interact with Twitter; to follow friends and post messages, pictures and locations. Some have more features than others, but most will offer more features than the Twitter website alone.
If you are new to Twitter, or want to give it a try, my username is acperkins. Feel free to send me a message.