A Stereo Pre-Amplifier is very similar to an integrated amplifier except it does away with actual amplifiers which are usually heavy, sometimes noisy and always hot.
The pre-amp takes in all your stereo (and mono) sources (including a phono stage for a turntable) and makes sure that the output signal is the purest it can be before the amplification stage.
This includes using the best possible DACs to handle digital signals. You will not find any fancy DSP (Digital Signal Processing effects) or Auto EQ on a Stereo Pre-Amp as it is intended for pure music reproduction only.
Of course pre-amps will require a separate amplifier to output the audio to the speakers.
Popular brands include Cambridge Audio, Arcam, Denon, Meridian, Pioneer and Onkyo.