Originally called police scanners, today's scanner radios receive much more than just police and emergency calls. You can listen to taxicab comapnies, utility companies, aircraft, military, amateur radio, family radio service, and sometimes even the space shuttle. Basically if a two-way radio is being used then a scanner can pick up the signal. A frequency is the number you program into your scanner to pick up who ever it is you want to listen to. It is their spot on the radio dial, just like 100.5 is the FM radio position of your favorite music station.
Scanners can pick up signals from as short as a few feet to thousands of miles.
Some signals actually bounce of the earth's atmosphere and can travel thousands of miles. Generally, expect the scanner to receive signals from a 25 to 50 mile radius from your location. Frequencies do not change often, but they do change. A wise decision would be to obtain a copy of a frequency directory yearly to keep up on any changes in your area. A bank is a grouping of channels.