What is compared
Activities (added, deleted and every field), durations, dates, float, constraints, progress, relationships, external predecessors (links from other projects - even ones you cannot open - with the date each link imposes, how far it moved, and the slack to your activity, listed every time so you can watch them), external dates (External Early Start / Late Finish: moved, slack, driving), resource assignments, activity codes, activity UDFs, notebooks, WBS, the calendars the project uses (hours, work week, exceptions), and project settings and scheduling options.
Durations tab: Original Duration for activities not yet started; once started, At Completion Duration (actual so far + remaining, worked out on the activity's calendar up to the data date), so you can see whether started work is growing or shrinking. Remaining Duration changes are on the Progress tab. Durations, float and lags are shown in days using each activity's calendar; date differences are calendar days. Left out: late dates (late start/finish, remaining late start/finish), resource assignment dates (they repeat the activity's dates) and edit stamps (who changed what, when).
A file with several projects: pick which one to compare. Relationships to activities in other projects are included, shown with that project's ID.