When I ask this question in training courses, most people just look at the screen blankly for a while and then take a peek at what their neighbors are writing.
You are the sender and you are sending a message. Anyone in the world can be the receiver. For some people, that is a hard thing to deal with. They can't imagine their audience.
So, think of it this way: someone in the world will be the receiver. Not everyone - just someone. So, start writing for the first person that will look at your page. Usually, that will be a friend or a colleague. Somewhere along the line, you are going to wander up to someone and say, "I've written a Web page. Wanna' have a look?"
Ask them what they think. Then, keep telling your friends and colleagues and keep getting feedback. Soon, you will be writing for a real audience.
There are two major rules to remember when you are dealing with the World Wide Web.
These two rules represent profound changes in the way that messages are published.
If you are interested in more information like this, I have drawn many of my ideas from Jakob Neilsen's Alertbox, one of the most authoritative guides around.