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document updated 16 years ago, on Mar 24, 2008

Impedance is the AC analog of resistance. (though unlike in DC, capacitors and inductors modify a signal's phase, and thus also impact impedance)

Impedance discontinuities cause AC waves to be reflected back towards the source. That one fact is why so much engineering time is spent on:

RF: standing-wave ratio (SWR) meters and impedance matching of transmitter → cable → antenna
line termination in long or very high-speed buses (ethernet/USB/PCI/etc) [parallel/series/Thevenin/AC]
worrying about the capacitance/inductance/resistance of transmission lines
for very high-frequency signals, being very careful about PCB routing (45°/mitred bends, minimizing stubs/spurs, minimizing vias), paying close attention to connectors, etc

In digital systems, the eye pattern is a great way to verify that appropriate margins are being held to.