From the rec.radio.scanner FAQ, written by Bob Parnass, AJ9S:

Every day and night, scanner hobbyists are entertained by what they overhear on their radios. Police cars, fire engines, ambulances, airplanes, armored cars, trains, taxis, and buses are all equipped with radios and you can listen to them. You can monitor the local sheriff and fire departments to hear about events "as they happen," before the news reporters hear about them. Hostage dramas, bank robberies, car crashes, chemical spills, neighbor and domestic disputes, tornado sightings are all fair game. In a single afternoon, you can hear a high speed police chase, Drug Enforcement agents on a sting operation, and undercover FBI agents as they stakeout a suspect.