The Move overlap section shows the moves both Pokemon learn, plus the moves unique to each. This accounts for every game in the series, so it's the full possible movepool, not just one game.
Step 1
Pick two Pokemon on the compare page.
Step 2
Scroll to the "Move overlap" section.
Step 3
'Both learn' shows moves they share in common, each colored by type.
Step 4
Below that, two columns list moves unique to each Pokemon. Great for figuring out which one can run a move the other can't.