REST is just a generally agreed upon convention that makes good use of HTTP verbs to create simpler URL structures when interacting with resources.

So basically, instead of having the following endpoints: site.com/create_user site.com/delete_user site.com/show_user site.com/update_user

You'd instead have a single use endpoint like so: site.com/user

And throw different HTTP verbs at it to signify the action.

It's definitely a well-defined concept and been part of Ruby on Rails since version 1.2.