Cell phones are basically combined radio transmitters and receivers that connect to a network of radio masts, which relay the signal between phones. Modern phones also contain small computers, allowing the phone to run software and store information, such as contact information and sound files.
Cell phones avoid the problem of colliding radio frequencies in a massive communication network by emitting a weak signal that is only sufficient to reach the nearest radio mast. The signal contains information that tells the network which phone is the intended receiver of the call. The mast transmits the signal to its base station, which sends it onward through the network. A base station and its associated masts are called a cell, which is the origin of the term "cell phone."
Calls between different companies, to landlines, and to other countries travel on a more complex route, often using communication cables at some point.
Early cell phone models used analogue technology, translating sound waves directly into radio waves. Modern phones are digital, which means that information is first transformed into a string of numbers and then decoded when it reaches its destination. One advantage of digital technology is that it makes eavesdropping more difficult.
Cell phones were first introduced in the 1980s, and they became highly popular around the mid-1990s. As of 2015, there are an estimated 7 billion cell phones around the world. In many Third World countries, they represent over 90 percent of all phone lines.