How do I tell a Twitter Tweet apart?

[gossipqueen]

I am having difficulty understanding who is saying what on twitter. When I go to someones twitter page if I look down at their tweet message board I see their name in bold next to their name in a lighter color, and below that I see another persons name. the name will say something like @someonehere and then a message beside it. Who is talking, the persons who's page it is or the persons who's name is next to the message? Or is the persons who's page it is replying to the person who's name is next to the message?

[Lee S.]

Whenever someone tweets a tweet to someone they have to use the "@username" so the one receiving it will get it.

So when you're on someone's twitter page, and you see the @sign, it means that person's sending a tweet to someone else.