A scanner is a radio receiver generally capable of picking up AM and FM (and sometimes SSB) radio signals anywhere from 100 kHz to 3.3 GHz. Popular amongst hobbyists, reporters, bounty hunters, prying neighbors, corporate spies, criminals and lawyers alike, scanners allow chosen frequencies to be stored in memory banks to allow them to be monitored later and will only stop 'scanning' when there is a signal strong enough to break the radio's squelch setting. Radios that do not have the ability to scan through frequencies are just referred to as receivers.
Radio scanners first became popular and widely available during CB Radio's heyday in the 1970s. The first scanners used crystals and had only a few programmable channels. Modern scanners allow thousands of frequencies to be stored in various 'memory banks' and can scan at a very fast rate due to modern micro-processors, they even allow scanning of the specific DCS or CTCSS code used on a specific frequency should it have multiple users. One memory bank can be assigned to overseas broadcasters, another can be for local marine communications and yet an other for local police frequencies that can be switched depending on the user's preference. Many modern amateur radio (transceive-capable) hand-helds (H/T's) are being designed with advanced wide-band scanning features for little more money than their receive-only cousins and are usually more ruggedly built. Lists of local frequencies are often available through the internet or local amateur radio clubs. A telescoping (extendable) antenna is a good idea for optimum performance if you are monitoring a wide variety of bands (longer for lower frequencies).
Radio scanners have historically been frowned upon by law enforcement agencies and the amateur radio community and championed by those supporting civil liberties. However, scanners are becoming obsolete in some communities as many local police, cellular providers, public service (and even amateur radio) agencies switch to trunked, digital, and encrypted radio systems. Although trunked and digital scanners have been developed, decryption capable scanners most likely would be outside the budget of the average consumer. Additionally, decryption of encrypted communications is a violation of U.S. law.
Under a law passed by Congress under the pressure of cellular telephone interests, a law enforced by the FCC, modern American scanners block the 800 MHz cellular band even though few cellular subscribers still use analog technology. Typically, there are Canadian and European unblocked versions available; these are illegal to import into the United States. The cordless phone bands at 46.000 - 49.000 MHz and 902.000 - 906.000 MHz can still be picked up by many scanners though. The proliferation of scanners led to cordless phone manufacturers going to a much more secure 2.4 GHz system using spread-spectrum technology.
In the United Kingdom and France, it is illegal to listen to almost anything outside amateur radio and mid-wave AM/FM stations in the broadcast band. Canada allows anything to be monitored but you may not disclose details of a conversation not broadcast to the general public. In the some parts of the U.S. there are extra penalties for the possession of a scanner during a crime. Other countries consider the mere possession of a scanner an offense.
Manufacturers include: AOR, Yaesu, Alinco, Radio Shack, Icom, Uniden, etc.