I find it best to think of AMP in two parts.
1) AMP is a subset for HTML that is build with speed in mind. You can't do everything, but what you can do is fast
2) AMP allow for use of AMP caches. Services like Google, twitter and Pinterest will keep a cached version of your AMP page that they can serve to their users "instantly" (ie, they preload)
Tis fast. I'm starting to see a few sites developed "AMP first" rather than having a separate AMP version. Not much adoption outside of CMSs with premade AMP themes though.