Audio Mastering is:
The last step before releasing your music (manufacturing, playing Live or DJ-ing); it ensures that the material is optimized for its final medium (digital, CD, vinyl…)
A mastering session includes: Bit/sample rate conversion (if required) ,equalization, stereo width adjustments, dynamic compression or expansion, loudening, spacing (pause between tracks), fades in/out, crossfades, levels (volume adjustment), removing clicks, pops & hisses, adding reverb…
Also you can read about audio mastering in Wikipedia
I provides digital mastering with some analog elements.
My DAW based on Power Mac G5 whith external plugins by TC Electronic, main monitors: Event ASP8, ADAT recording, hi-end comutation, vacuum tube delicate saturation if needed, and Individual approach to music and concept.