Oscilloscopes show the voltage of a signal over time, as a curve or trace. It is one of the most useful measurement instruments in an electronics lab, for both digital and analog work.
Oscilloscopes often have two inputs which can be shown simultaneously. In this way one measure e.g. input and output signals side by side. More advanced oscilloscopes may have four or even more channels.
Modern digital oscilloscopes often have a range of other measuring functions, such as frequency measurement, rise and fall times, voltage, etc, shown alongside the curve of the signal.
Some digital oscilloscope can do FFT, i.e. mathematic frequency analysis of a signal. These oscilloscopes can be used as simple spectrum analysers.