Logic Gates

 A logic gate is an elementary building block of a digital circuit.  Most logic gates have two inputs and one output.  At any given moment every terminal is in one of the two binary conditions low (0) or high (1), represented by two voltage levels.  The logic state of a terminal can, and generally does change often, as the circuit processes data.  In most logic gates, the low state is approximately zero volts 0V, while the high state is approximately five volts positive (+5V)  Starting Electronics Construction, Keith Brindley

introduction to logic gates

After learning the basics about logic gates, here as a full list of them with their corresponding truth tables and how to substitute logic gates for  another type, there is also details of how to make any gate from a NAND or NOR gates:

full list of logic gates and truth tables