This web page does not use up your data, drain your battery, or spy on you. It doesn't assume that you're sighted or on a first-world device or Internet connection. It doesn't try to control your visit or keep you locked in. It is not an app.
Web apps often try to keep you there instead of connecting you with other sites. There is often no way for other sites to create links to particular parts of an app.
Web pages like this are accessible in several senses. It is just a structured text document -- hypertext -- so it can be understood by many automated tools in addition to well-known browsers like Firefox:
Web pages like this are also accessible to people with low-performance devices and slow or unreliable internet connections. They don't load lots of resources from lots of places nor execute complicated programs.
Web apps often dynamically load the text they display, making it harder for people's tools to understand them. They often download large amounts of data to your device and require significant computing power and electricity to use.
On web pages like this, what you see is what you get. It doesn't run programs on your device in the background for other people's purposes. It doesn't track or identify you, animate or pop-up to capture your attention, or so on.
You can use browser extensions or other tools to change this page. Maybe you prefer large text or high-contrast colors. Maybe you want everything to be pink. Ok! You can use your own keyboard shortcuts, browser forward/back buttons, etc.
Web apps often dynamically load and run programs from many sources. The purpose is often to control the user experience, not empower the user to shape their own experience. The purpose may be to exploit the user or gain something from them, without transparency or meaningful consent.
When you visit a site, it gives your device some information in an HTML file. If it's a webpage, then it's just a text file in a format your browser can understand and display for you. If it's a web app, then it's an whole interactive program or application running on your device.
Most sites today are actually a mix between a webpage and web app, having aspects of both. By being aware of the differences, we can make conscious, informed choices when we build and visit sites.
Unfortunately, often people only need a webpage but they build a web app instead. Maybe they want the flashiest, coolest-looking page possible. Maybe they don't know about the differences. Maybe their company thinks these differences are okay or even good for business. Or maybe they're just following the development trends encouraged by such companies.
By default, web apps try to control their users' experiences. Webpages try to provide their visitors with material to use as they like.