der Unterschied

difference from sb. / sth. (usually 'im Unterschied zu')

what it differs from — a single reference point, not the pair used with 'zwischen'

Im Unterschied zu ihm interessiert sie das überhaupt nicht.

Unlike him, she is not interested in that at all.

wozu?dazu

difference between sb. / sth.

between what the difference lies — two things compared side by side

Der Unterschied zwischen den beiden ist klein.

The difference between the two is small.

wozwischen?dazwischen