Relays are for switching, only thing we need to know is what type of relays can be used as single pole single throw, SPDT, DPDT...., and also the voltage consumption as where to use in the field of power electronice, analog , microwave, this way differs
Relay is not that very reliable element in microelectronics where you can build better switches by other means and faster. Relays better are understood in automotive applications.
amjad mentioned word microwave. Relays are not that very much at all used in microwave. Ability of microwave to switch on/off is very slow comparing speeds microwave circuits want.
Omron relay(www.omron.com) is good. But standard relay have bandwith too small. For high speed apllication I suggest use semi_relay that faster and reliable