Transistor-Transistor Logic - A technology used to produce logical functions, which uses transistors to provide those functions.

A popular series (or "family") of TTL logic integrated circuits is the 74xxxx ("seventy-four hundred") series. This series of chips also has a military grade implementation that is numbered exactly the same, but with a '54' prefix in place of the '74' prefix.

More recently, the 74xxxx series of TTL logic chips have been re-implemented using CMOS technology. The series is named 74HCxxx. The 'HC' signifies "High-speed CMOS."

The chips in the HC series have the exact same functionality and pin-outs as the TTL series, but with the advantages afforded by being implemented in CMOS (such as lower power consumption and very high input impedance).