Under review is first. On hold is an outcome from being reviewed. Others include rejection, interview, or silence (which looks the same as 'under review', which either suggests they haven't looked at it, haven't decided, or have silently rejected). Each school treats applicants differently, so I honestly wouldn't hurt yourself trying to read the tea-leaves.
Agreed. "Under review" means that someone is looking at your application and deciding how they feel about you. "On hold" means they've decided...that they don't dislike you enough to reject you, but that they aren't impressed enough to interview you right away. A hold can transition to either an interview or a rejection as the season progresses.