Twitter blog explains the tracking twitter:
When someone (anyone who updates in public) mentions "NYC," you'll get it on your device in real-time. From there you can send "whois username" to find out more about that person, or "follow username" to follow his or her updates. Don't want to receive anymore about NYC? Toggle it off with:
You can create as many of these as you want, so send "track drinking tea", "track iphone", "track walking san francisco" and you'll receive matches for all. Want to get a list of what you're currently tracking? Send "track" alone (or "stats"). Turn them all off by sending "track off".