Cell phones that have the ability to run the Google Maps application can broadcast their location so they can be tracked. This is a feature called Google Latitude and it requires both you and the other person to accept the tracking. You cannot track someone's cell phone without that person's permission. Google Latitude overlays the location of you and your friends in Google Maps. If the phone being tracked doesn't have the ability to use global positioning satellites, Google can still track the general location by using cell towers.
Browse to google.com/latitude and log in to your Google Account. Your Google Account is the same login that you use in Google services like GMail and Google Docs.
Click the "Add Friends" link on the left side. Click on the drop-down menu at the top of the dialog box and choose "All Contacts." Scroll through the list until you find the contact information for the person whose phone you want to track. If there is a blue icon next to the name, that person is already using Google Latitude. Click the "Add" link to add that friend. The tracking will not begin until that person accepts your request.
Click the "Friends" tab on the Google Latitude page; you'll see a Google Map on the right. To the left of the map is a list of your Latitude friends. Click on the name of your friend in the list to see where that person's cell phone is on the Google Map. You can also see how long it has been since the location was last updated.