Terror is usually described as the feeling of dread and anticipation that precedes the horrifying experience. By contrast, horror is the feeling of revulsion that usually follows a frightening sight, sound, or otherwise experience.
While both genres rely on suspense, in a thriller, it's far more important that the audience can't tell what's about to happen next. In contrast, horror directors often make it painfully apparent that a character is about to meet a gruesome end.