Back in version 1.2 or so, Rails introduced/massified a concept known as RESTful. That's appropriately named because RESTful isn't REST It's REST-like, and that's a good thing. Pure REST is a pain in the ass anyway.
Yes, APIs existed before that, REST or otherwise. Rails however redefined the landscape, and heavily influenced API design afterwards. Along with REST, it made popular the notion of pretty (or human as I call it) URLs. It went like this:
POST /books [data]
That creates a book with whatever POST [data]
you're sending. It makes sense. You're "posting a book", which isn't far away from how humans understand what happened. Continuing:
PUT /books/1 [data]