Scanning receivers and accessories.
Scanners are wide band radio receivers that cover a range of frequencies. Most have "gaps" in their coverage and generally the more you pay the fewer gaps they have. The main gap that the cheaper scanners have is the UHF Military Airband.
The frequency range of different models can range from less than 1MHz to over 2GHz, however the majority of scanners are designed to cover the VHF & UHF bands and typically cover from around 60MHz to 500MHz.
It is important to decide what you want to listen to before buying a scanner - it is false economy to buy a cheaper one only to find later that it misses out some frequencies that you would like to listen to. Ideally make a list of all the frequencies you are likely to want to listen to and then see which of the scanners covers all of them.
In addition to the frequency selection the more expensive scanners often allow you to switch the mode between AM & FM. On the cheaper ones this is often done automatically and you can not change it.
Many of the latest scanners can also be programmed by computer - this makes it much easier if you need to enter a large number of frequencies, it can also allow you more control over some of the scanners functions. Usually you need to purchase a lead and software separately.
Some of the bands covered by scanners are: Aircraft, Marine, Amateur Radio, Taxis, 2-way radios, Motor Sport, etc.
None of the scanners will pick up the police or any emergency services.