Both are situational judgment tests designed to assess professionalism and interpersonal decision-making rather than academic knowledge. Casper, administered by Acuity Insights, uses a mix of video-based and typed-response scenarios. PREview is the AAMC's own version of the same general concept.
Whether you need either one depends entirely on your specific school list, since requirements vary by program and change from year to year. Check each target school's current website before the cycle rather than assuming last year's requirements still apply. Prepare by practicing the format and pacing, not by trying to memorize "correct" answers, since these tests are explicitly designed to resist that approach.