Sails is, of course, a web framework. But take a step back. What does that mean? Sometimes, when we refer to the "web", we mean the "front-end web." We think of concepts like web standards, or HTML 5, or CSS 3; and frameworks like React, or Vue.js, or jQuery. Sails is not "that kind" of a web framework. Sails works great with React and Vue, but you would never use Sails instead of those libraries.
On the other hand, sometimes when we talk about "web frameworks", we mean the "back-end web." This evokes concepts like REST, or HTTP, or WebSockets; and technologies like Java, or Ruby, or Node.js. A "back-end web" framework helps you do things like build APIs, serve HTML files, and handle hundreds of thousands of simultaneous users. Sails is "that kind" of web framework.