Repeater is an electronic device that receives a signal and retransmits it. Repeaters are used to extend transmissions so that the signal can cover longer distances or be received on the other side of an obstruction

Digipeater: A repeater node in a packet radio network. It performs a store and forward function, passing on packets of information from one node to another.

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  Amateur radio repeater: Used by amateur radio operators to enable two-way communication across an area which would otherwise be difficult by point-to-point on VHF and UHF. These repeaters are set up and maintained by individual operators or clubs, and are generally available for any licensed amateur to use. A hill or mountaintop location is a preferable location to construct a repeater, as it will maximize the usability across a large area.
  

Radio repeaters improve communication coverage in systems using frequencies that typically have line-of-sight propagation. Without a repeater, these systems are limited in range by the curvature of the Earth and the blocking effect of terrain or high buildings. A repeater on a hilltop or tall building can allow stations that are out of each other's line-of-sight range to communicate reliably.^[5]
Radio repeaters may also allow translation from one set of radio frequencies to another, for example to allow two different public service agencies to interoperate (say, police and fire services of a city, or neighboring police departments). They may provide links to the public switched telephone network as well,^[6][7] or satellite network (BGAN, INMARSAT, MSAT) as an alternative path from source to the destination.^[8]
Typically a repeater station listens on one frequency, A, and transmits on a second, B. All mobile stations listen for signals on channel B and transmit on channel A. The difference between the two frequencies may be relatively small compared to the frequency of operation, say 1%. Often the repeater station will use the same antenna for transmission and reception; highly selective filters called "duplexers" separate the faint incoming received signal from the billions of times more powerful outbound transmitted signal. Sometimes separate transmitting and receiving locations are used, connected by a wire line or a radio link. While the repeater station is designed for simultaneous reception and transmission, mobile units need not be equipped with the bulky and costly duplexers, as they only transmit or receive at any time.
Mobile units in a repeater system may be provided with a "talkaround" channel that allows direct mobile-to-mobile operation on a single channel. This may be used if out of reach of the repeater system, or for cRepeater could also be a network device that retransmits the data from the sender to the receiver side of the network. The Incoming data are often within the type of wireless, electrical signals, and optical signals. The Repeaters transfer the data through an outsized area distance. 
The transmission of signals makes them somewhat weaker and. This process is known as Attenuation. Repeater could also be a quite networking device that accepts incoming signals from routers and strengthen them so that the signals are often retransmitted to the devices. A touch sort of a hub too works at the physical layer of the OSI model.
Generally there are 3 kinds of repeaters. Those are the phone repeater, radio repeater and optical repeater. Repeaters are especially useful when signals are being distorted because of transmission loss.