[jdcfsu]

Retweeting is used to send something someone said on your follow list to everyone else. I think it's stupid because if everyone wants to know what that person says, they'd be following them in the first place.

Replying is a way of directly responding to something someone says. So if you like something I said, or want to make a comment or answer a question, then you use @username and respond to them. It's a way of creating a threaded conversation out of chaos. If you reply to someone and they don't follow you, it won't show up on their main feed but it will show up in their 'mentions' tab. So it depends on the user and how they use the service. Some third-party apps will display all 'mentions' inline in the feed, but it depends on that app and the settings there.

Stick with it. It took me a few days of using Twitter heavily to really get the hang of how I wanted to use it. It's got a little bit of a learning curve in that there are literally a million different uses for the system.