ReplayGain is the name of a technique invented to achieve the same playback volume of audio files. It specifies the reference level of 89dB and an algorithm to measure the perceived loudness of audio data.
ReplayGain is different from peak normalisation. In peak normalisation, you merely ensure that the peak amplitude reaches a certain level. This does not ensure equal loudness. The ReplayGain technique measures the effective power (i.e. RMS after an Equal Loudness Filter) of the waveform, and amplifies the waveform accordingly. The result is that ReplayGained waveforms are usually more uniformly amplified than peak-normalised waveforms.
Foobar2000 is the best Windows tool for using ReplayGain on files as it handles many different formats. It can apply the loudness changes within tags or it can even irreversibly apply them direct to the audio file itself. The latter means the audio file will be played at the new loudness on all media players.