The sound editor is responsible for assembling everything you hear on screen — the dialog, the foley, ADR (automated dialog replacement), walla (crowd noises), incidental sounds (paper rustling), atmosphere (wind, a distant tugboat) and sound effects (engines revving, gunshots). Sound editors make audio choices; sometimes they tap into a library of sounds, sometimes they make their own recordings, and sometimes they fabricate sounds that are completely new.
This is one of the final stages of post-production. The sound elements have been prepared/gathered in isolation, and the sound mixer takes all of these (plus the music score), and determines the appropriate levels, judging which elements the audience needs to hear. In other words, after the sound editor has assembled what the audience hears, the sound mixer determines how they hear it. The mixer decides when to de-emphasize atmosphere sounds and prioritize music, for example.
A competent sound analysis will also remark upon the character of sounds (ex. rough, muted, busy, etc.).