Depending on what characteristics of a signal or a system you need to measure you may be able to use standard instruments, but you may need to use specialized (and expensive) instruments. The equipment listed and described below covers most of the equipment you might use.
An oscilloscope measures the voltage of a signal with respect to time. As such it is used for time domain analysis of signals. Oscilloscopes are rated by how high of a bandwidth they can measure, the higher the frequency the scope can measure, the more complicated and expensive the electronics become.
A spectrum analyzer measures the frequency components in a signal. It displays the power of the signal at different frequencies on its output screen.
A network analyzer does not refer to measuring traffic on a computer network, it refers to measuring signal characteristics on an electrical network. Network analyzers usually measure very high speed electrical networks and characterize the network by measuring strengths of incident waves and reflected waves as signals travel through the network.
Time Domain Reflectometer
A time domain reflectometer (TDR) is used to determine cable length or to determine the distance to a break in a cable. The TDR is very useful for finding cable faults in buried cable whether it’s copper cable or fiber. The TDR works by sending out a pulse on the cable and timing how long it takes for a reflection to return to it. Whenever there is a change in the nature of the cable (i.e., at a break or at a connector), some (or all) of the signal will be reflected instead of passed through. By timing how long it takes to get a reflection and using the speed of the signal through the cable, the distance to the reflection point can be determined. For more information, see this in depth page on transmission lines and TDRs.