Twitter is used in many different ways.
As @dulwichmum says, its nearest equivalent that people are familiar with is Facebook status updates.
Twitter updates, or “tweets” as they are sometimes called, are limited in length to 140 characters.
Apart from that, there are no rules about how to use Twitter.
Some people use it to share ideas, thoughts and links or to ask questions.
You can choose whether to make your updates public or private.
If you think someone is saying something interesting you can “follow” them, which subscribes you to their updates, which will appear on your Twitter homepage.
You can see who other people are following and who’s following them.
Companies use Twitter to send out updates about their products and services and answer customers’ questions.
Even Tower Bridge has an account on Twitter, which sends out a tweet whenever the bridge is opening or closing.
There is a growing number of Twitter tools available to write, read and manage your updates, including apps for the iPhone.
One of the first and still one of the best applications I’ve seen is Twittervision – which overlays tweets from Twitter on a google map of the world, so you get to see where people are when they tweet something.