A cable tester is a device that is used to test the strength and connectivity of a particular type of cable or other wired assemblies. There are a number of different types of cable testers, each able to test a specific type of cable or wire (some may be able to test different types of cables or wires). A cable tester can test whether a cable or wire is set up properly, connected correctly, and the communication strength between the source and destination. The picture is an example of a cable tester from TRENDnet.
For computers, one of the most common types of cable testers used is for testing Cat 5, Cat 5e, and Cat 6 network cables. Because so many different types of data can be transmitted over a network cable, it is important that the network cable connects properly between the computer and server. Also, make sure the signal strength between computers is adequate for transmitting data and that there is no outside interference causing a loss of data or decrease in signal strength. A cable tester can test for these factors and verify the network cables connections are correct and will work for the intended purpose.