Mechanical push buttons use a metal terminal that physically moves to connect or disconnect a circuit. In some cases, a spring is used to maintain that the button returns to the normal position after a press event. The figure below shows how snap domes can create mechanical motion and electrical connection, as well.

Figure 2-1 Snap Dome Stackup Example

One common problem with these mechanical designs is wear and tear. As the mechanical contact bridges the electrical connections, the metal can wear down as the electrical connection is repeatedly connected and disconnected.