This website--flightview.com--is a one stop shop for checking on the status of any flight on any airline flying in the US. This site keeps track of just about all commercial flights within the US and Canada--flights headed for a chosen hub get a special color and airplane icon, and the hub switches from time to time. How does the site keep track of sometimes over 3000 flights at one time? Radar data--both air traffic and weather radars--keep track of whether a flight is on course, it's speed and flight level (or altitude), plus any meteorological obstacles (like rain and thunderstorms the plane would have to fly around or over).
To track a particular flight, you have two options: with the first you choose an airline and flight number which will give you the current flight information. With the second option, click on the "track your flight by departure and arrival cities" link and then fill in the blanks including approximate departure or arrival time. Click "OK" and you'll get a list of flights that fit that description. Then click on your flight to get its information.