Mastering vs Mixing

 A crystal clear AD/DA converter with professional (preferably passive) 3-way midfield speakers are used in Mastering so as to reflect all frequencies to mimic a typical home listening situation. Nearfield monitors are best suitable for recording and mixing studio (for precise instrument localization and reverb/effects judgment). 

Analog high-end processors are used in Mastering for best sonic results ( quality, noise, headroom) . The gears are often custom-made and carefully chosen due to each Mastering engineer's taste (in tube sound and other technical features). 

Digital plug-ins are rarely used in Mastering (only a few limiting is needed) whereas Mixing nowadays relies on digital processing (especially in home studios) with the settings and all tracks are recalled by a single click within the session.

A special DAW is used for Mastering with an extended algorithmic calculation and other features ( ISRC and PQ codes, CD-Text, DDPi delivery, formats...). Other DAWs are more suitable for mixing/editing. 

Early reflections in Mastering are kept at minimum for a precise judgment without any barriers between the monitors and the engineer (an inclined ceiling is a plus). We often see a large console or desk in mixing studios which is important and practical (but has its drawbacks...).

A dedicated room treatment with different specs (NC, ER, Reverb Time) is needed for Mastering. The mixing room is often surrounded by machines/fans/AC. Mixing is sometimes done in a live concert with PA system (or even bedrooms!)

 Mastering Engineer is trained through years to listen in a macro  perspective, concentrating on the whole stereo track as one group. Whereas the Mix Engineer is trained to listen more in a micro perspective, concentrating more on each track, the balance between each track and group, and the stereo image. 

A fresh set of Healthy ears with a creative mind are essential  in Mastering to snatch any minor issue since this is the last stage before releasing the Music!