A relay is a switch that essentially controls a large current with a small current; usually a much smaller current. There are two basic methods of accomplishing this. The electromechanical switch approach is a very reliable and mature technology that has been around for about 150 years, but it is also mechanically complex, involving many moving parts that wear out. The other is solid state technology, which is a much more reliable solution that doesn't rely on moving parts and also offers the bonus of having a faster response time. There are solid state switches such as SCR triacs for AC-only solutions or MOSFETs which can be used for both AC or DC solutions. However, switches provide switching only; not isolation.