it's easy to tell what is free and what is not as the movie poster has a "prime" banner on it if it is part of prime. plus search feature has option to only show prime results.
you also get prime music, books, audio books, delivery with full membership. they work the same and offer a large chunk for free with others pay as you go. though the music part has been seriously nerfed to get folks to pay extra for music service. but still a lot to stream for free on it.
What most of the answers have described is Prime Video. I think that's $35 or $60 a year? I don't recall and can't find it on Amazon's site (they make you go through the steps of actually subscribing to see the price). Yes it's like Netflix.
Frankly, except for a few TV series and older movies, the selection of Prime Videos is not very good. I don't think Prime Video is worth it even at $35. But the full Amazon Prime membership can be worth it especially if you use several of the offered services. Free 2-day delivery is the big one. But after Prime Videos and Prime Music, Prime Photos (cloud backup of your photos, unlimited number and size) is also very useful and little-known. If you own a Kindle device, the library is good too.
prime video is actually $8 a month ($96 a year) by itself!! which makes it rather silly to get since for $99 a year you get the full prime membership. students can get prime for half price i believe it is. something like 3 months free and then half price for the year.