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The difference is the same as with any aggregation function (MIN, STDEV, AVG) and table calculation (WINDOW_MIN, WINDOW_STDEV, WINDOW_AVG) that is aggregation functions operate on underlying records (SQL) while table calculations on aggregated measures as per your sheet's dimension pills arrangement and partitioning/addressing setup.
You might start with reviewing the online training videos related to Table Calculations here: Intro to Table Calculations | Tableau Software. There are a few additional videos in that series that should help as well.
Table Calculations and Level of Detail Calculations bring a lot of options and power to your Tableau skills. If you come to Tableau Conference, be sure to check out the advanced training sessions that include these features.