Review (schema.org)
The schema.org type Review allows individual reviews to be marked up in structured form and can lead Google to display them as a rich snippet with a star rating, author name and review text.
In practice
The precondition is that the reviews are genuinely visible on the page and were not invented, bought or generated automatically, because Google penalises fake markup of this kind. Review combines well with Product, LocalBusiness or Service; for an overall picture of several reviews, AggregateRating is a sensible addition.