Thanks to Yarik Halchenko for this explanation.
Imagine a series of commits A, B, C, D... Imagine that there are two branches, topic and main. You branched topic off main when main was at commit 'E'. The graph of the commits looks like this:
A---B---C topic
/
D---E---F---G main
Then:
git diff main..topic
will output the difference from G to C (i.e. with effects of F and G), while:
git diff main...topic
would output just differences in the topic branch (i.e. only A, B, and C).