Repetier-Server runs on Windows XP and later versions, Max OS X 64 bit 10.10 or newer, linux 32 and 64 bit for supported linux distributions like Ubuntu, Debian, Mint, Raspian and other Debian like distributions.
The server has to be connected to a printer with serial interface.
The hardware requirement for the pure server is very low, it already runs on 256 mb RAM like Raspberry Pi. Some functions like slicing may need more resources, but the server can outsource such computations to other instances running in your network.