Question about someone's twitter page?

[Alex]

Ok, I don't know much about Twitter so I have a few questions.

When I visit someone's twitter page, is it only that person's tweets, or do other people's tweets about that person show up on there as well?

And what does it mean when certain messages have " @*username* " in it?

[sandra]

I suggest you check out this website, it's a Twitter guide and can be really useful for people who are new to Twitter: http://business.twitter.com/twitter101

But to answer your questions...
When you visit someone's Twitter page, you can only see that person's tweets.
When a tweet includes @username it either means that a person is sending a message to the @username or just reffering to them in their tweet.
Examples (taken from the before mentioned website):
This would be a message to @Ev: “@Ev Glad you liked our vegan cookies. Thanks for twittering about ‘em!”
This is would be a tweet referring to @Ev: “Just made a batch of vegan cookies that @Ev really liked”