The JAMstack is a modern web architecture: helping people create fast and secure sites and dynamic apps with JavaScript, APIs, and prerendered Markup, served without web servers. This means Wordpress, Squarespace, Drupal, and other server-run web apps are not JAMstack (sometimes I find it helpful to define things by what they are not). Chris Coyier recently wrote a great post about how it is all about static asset hosting, and you can use as much or as little of J, A, or M as you like.
Netlify serves the JAMstack developer - you don't have to use Netlify to be JAMstack, but we just happen to be very very very very very very very very focused on those needs and provide all the tooling you might need including form handling, authentication, and serverless functions, as well as building on top of the JAMstack assumption by providing continuous deployment with instant rollbacks and branch deploys, free SSL (HTTPS) on Custom Domains, split testing, image transformation, and local development.