Answer:
Electrical signals are either direct current (DC) or alternating current (AC).
For DC signals, information is carried by the existence of a DC level, or it's absence. This is binary signalling, and communicates either a '1' or a '0'.
An AC signal conveys information by either changing one of the three properties that AC signals have:
1) changing the amplitude of the signal (this is amplitude modulation, like an AM radio)
2) changing the frequency (this is frequency modulation, like an FM radio station)
3) changing the phase of the AC signal. This is used in the faster of the old-style computer modems, as well as DSL modems