What is difference between regenerator and repeater?

Dear Ahmed In DWDM, we always distinguish between two functions which are REGENERATORs and AMPLIFIERs. Regenerators detect the signal by converting it into an electrical signal then re-transmit it again as a fresh copy by performing the3R function (re-amplifying, re-shaping and re-timing), while amplifiers only amplify the signal with the noise. Repeaters are not that relevant to DWDM but you can consider them as the regenerators. After multiple amplifier sections you will need a regenerator to regenerate a clean signal without noise and other affects of fiber impurities.

Repeater is simply amplify the Transmitted signal & Re-generator is used support  3R ,1-Reshape2-Retime3-Regenerate The signal as it transmitted from source end.

Repeater amplifies the whole signal both signal and noise.  SNR will not be improved. But Regeneration is recreating original waveform depending received signal. SNR will be improved or it is as if new signal is being transmitted.  

Repeater and Regenerator their function is basically to "boost" the signal, so that signal can cover longer distance without attenuation.

but the regenerator includes circuitry to recover the clock timing information, which is then used to determine when the output signal switches its state. This ensures that the recovered data from the threshold detector is adjusted to provide the correctly timed signal output.