Digital circuits are constructed from electronic switches. The switches are either ON or OFF; there are no half-way states so the only voltages present in a digital circuit are the supply voltage V_s (usually +5v) and zero. If a point in a digital circuit is said to be "high" we mean that it is at a voltage very nearly equal to V_s. If the point is said to be "low", it means its voltage is very nearly zero. A high state is represented by "1" and a low state by "0".

Logic gates are special switches. One logic gate by itself is not much use. When many gates are connected together, they become very useful; a computer, for example, is basically made from thousands of gates (and other types of special switches).

All the gates in the kit (except the inverters) are two-input gates. The voltage which appears at the output depends on the voltages applied to the two inputs. A truth table is a table which shows how the output is related to the inputs.