Question about someone's twitter page?

[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”