Oscilloscopes and waveform generators
An oscilloscope is a device that lets you analyze electrical signals. Its use is often indispensable when designing, debugging or simply using electronic circuits.
A signal generator is a device capable of producing electrical signals (typically periodic signals), the characteristics of which can be specified by the user. The main characteristics of a periodic signal are waveform, amplitude, and frequency.
Tektronix (a major oscilloscope manufacturer) publishes a good introductory guide to signals and oscilloscopes.
Beware! Especially with high-frequency circuits, you definitely 'cannot' ignore the fact that oscilloscope probes are not ideal (i.e., they have capacitance and inductance as well as resistance). You could even destroy your electronic circuit pretty easily. Before it's too late, read this article (useful and funny!).