The difference between RCV (also called IRV) and STAR is the difference between an Ordinal system and a Cardinal system.
An Ordinal system is a ranked system. Chose one or the other, but never both. A vote for A means you cannot also support B. This lead to some math shit that actually gives preferential treatment to two party systems.
RCV claims to support third parties and solve the spoiler effect. The truth is the opposite in every way. It eliminates fringe parties that would spoil elections, but also falls prey to spoiler effects when you have very similar candidates. It's actually a mess.
STAR on the other hand is a Cardinal voting system. A vote for A is a vote for A and a Vote for B has no impact on A. A good example is saying that I give Chocolate Milkshakes 5 out of 5 stars and New Coke 1 out of 5. But here's the main difference to an Ordinal system, I can also give a Banana Smoothies 5 out of 5 stars. Because I'm rating them as individuals, not in comparison to each other.
STAR is literally a 5-star review of the candidates, and the two with the highest average (or just highest scores) are then put head to head. Each ballot is then looked at, if Chocolate Milkshakes are rated higher on any given ballot than Banana Smoothies, Milkshakes get the vote of that person. If they're the same, a vote of No Preference is logged, and the No Preference votes are also made public at the end.