How do the police use mobile/cell phones to trace us?

I heard that your personal movements of place and time can be placed by tracking your mobile/cell phone. Do the police have access to everyone's phones? If they don't know their number how could someone be tracked, and how accurate can someone's location be pinpointed? I mean it tracks them by bleeping to the nearest antenna or something, so within how many meters ca someone's location be pinpointed?