There's some variation from game to game, mainly with cultists, but generally the difference is this-what's the difference?
are they both none-wolf baddy roles that don't hunt, can attach to a pack, and are scanned by priest and not seer?
Cultists can start pack-aligned or unaligned. They don't hunt, lose if there are no wolfs left, and can usually switch pack allegiances once all wolves in their pack are dead. They can be scanned by priest/sorc scans.
Zealots always start attached to a pack, don't hunt, and when all wolves of their pack die, they become wolves. While zealots they can be scanned by priest/sorc scans.