Spy phone recorder is a term used to describe any device designed for secretly recording telephone calls. Some of these are plugged into another telephone jack on the same line, and they will automatically detect when an incoming or outgoing phone call happens. At that point, they will come on and begin recording until the phone call ends. Sometimes they may be designed with special features to keep track of the timing of phone calls and other potentially useful data. Another kind of spy phone recorder is actually built into the phone itself, and it could be installed in a specialized standard telephone or a cell phone.
People have very different reasons for wanting to purchase a spy phone recorder and document telephone conversations. One common reason is to criminally implicate someone. For example, if someone is making threatening telephone calls or prank calls, recording them could potentially be useful. Some other people may record telephone calls just to keep a record of a phone conversation so that they can remember or prove what was said. Some people do this for purely informational purposes or as a precaution in case someone wants to lie about a phone conversation.
The legality of using a spy phone recorder can vary significantly in different places. Some places have no laws against taping people, while others may have major restrictions. For example, in some places, taping is legal as long as at least one of the callers is aware and consenting. In other places, it's mandated that both parties are aware of the taping. There are also places where it is totally illegal for civilians to tape calls under almost any circumstances.
Sometimes people record phone calls to prove something in court. This can sometimes be a waste of effort because many places won't allow this kind of evidence. It is generally recommended that someone check with a lawyer before even attempting to use a spy phone recorder as a legal tactic.
In terms of technology, a spy phone recorder may use several different kinds of media to record the conversations. Some versions use tapes as the recording medium, while other devices use electronic flash media or hard drives. They can also vary a lot in terms of the quality of the audio on recorded conversations and special extra features.