Four-way switches have four terminals. This type of switch works in combination with two three-way switches to control electricity to lights from three (or more) locations. They support hot conductors (traveller wires), which receive and transfer electricity from each of the three-way switches.
A 3-way switch will usually have 4 screws. Most older 3 way switches only have 3 screws. The difference between the older and the newer switch is, the ground screw. Code changes over the years have required the addition of the ground screw for safety reasons. However, it is not needed for the correct operation of the 3 way switch. It is needed for the safe operation of this switch or any other type of switching device. On some switches the colors of the screw terminals are different to differentiate between the common and traveler screws. Some switches are made differently from others so the colors used may differ between manufacturers. The common screw will nearly always be a different color from the traveler screws.
A simple on-off switch: The two terminals are either connected together (allowing current to flow) or disconnected from each other, breaking the circuit, as you throw the switch.
Sometimes referred to as a Single pole, single throw switch (abbreviated to SPST). May also be referred to as a toggle switch.