Copy
copy() duplicates a finished solid at several positions, in a row or around an axis. The copies are clones of the shape as it is — no modelling operation is re-run. To re-apply a feature (a cut, an extrude) at each position instead, use Repeat.
Repeat
repeat() re-applies a feature at several positions. The feature — a cut, an extrude, a fillet, a rib — runs again at each new location and interacts with the solid there, exactly as if you had modelled it there by hand. A repeated cut gives one solid with many pockets; a copy of the same cut would give many solids.