The line matters more than it sounds — it determines cost, timeline, and what kind of team you actually need.
People use "website" and "app" almost interchangeably in casual conversation, but the distinction matters a lot once you're scoping a project — the two are built differently, cost differently, and solve different problems.
A website presents information to a visitor. A web application lets a logged-in user do something — manage data, view a personalized dashboard, process a transaction. If nobody ever logs in, it's a website, however sophisticated the design.
A restaurant's menu and hours page is a website. A restaurant's online ordering system, where a customer creates an account and tracks an order, is a web application layered on top of (or next to) that website.
Websites are largely about content, design, and search visibility. Web applications add data models, user accounts, permissions, and business logic — an entirely different set of engineering concerns on top of the visual layer.
This is also why a web application generally costs more and takes longer than a comparable-looking website: the parts a visitor doesn't see (data storage, access control, business rules) are often the majority of the actual work.
A common, sensible path is a marketing website first (to establish presence and generate leads), with a web application added later once there's a clear need — a customer portal, a booking system, an internal tool. Building the website on a foundation that can support that later addition avoids a costly rebuild when that day comes.