Web app (web application) is software that runs in the browser so someone can do a job: book a room, edit a document, manage orders, pay an invoice. A brochure site explains the business. A web app is a tool people return to.
It is not the same as SaaS. SaaS is a way of selling software, usually as a subscription. The product is often a web app, but a web app can also be internal and free to staff. A PWA is a web app or site that can be installed and feel closer to a phone app. Cross-platform is a different bet: one codebase that ships a store app to iOS and Android. A native app is that store app written for one platform only. Front-end development is what people tap. The backend holds the data. Hosting keeps it online. An MVP is often a thin web app built to test product-market fit. A URL there opens a page; inside a phone app, that address is a deep link.
A real-life example of a web app
A Midtown clinic’s website lists services and a phone number. That is a site. The page where patients pick a slot, fill intake, and get a confirmation is a web app — even on the same domain. If that scheduler is slow, or form validation never explains a bad phone number, the pretty homepage does not save the appointment.
Build the task, not only the brochure. Read: website vs web app and what type of website you need.