"Website" and "web app" get used interchangeably, but they are different things with very different costs. Knowing which you actually need keeps you from overbuilding a brochure or underbuilding a tool.
The simple distinction
A website mostly presents information: pages you read - about, services, contact, a blog. A web app lets people do work: log in, enter data, and get something back that changes based on what they do. If the screen mainly shows, it is a site; if it mainly does, it is an app.
When you need a website
When your goal is to be found, explain what you offer, and get people to contact or buy. Marketing sites, portfolios, and content hubs are websites. They should be fast, clear, and easy to update - not overengineered.
When you need a web app
When people need to accomplish something interactive: a customer portal, a booking system, a dashboard, an internal tool. Anything with logins, records, and logic that responds to the user is an app - and it carries the cost and care of software, not a page.
The grey area
Plenty of projects are both: a marketing site with a customer portal attached, or a content site with an interactive tool. The trap is building the whole thing as an app when only one corner needs to be. Scope the interactive part precisely and keep the rest simple. Explore websites and web apps.
Describe what people will do on it and we will tell you whether you need a site, an app, or a little of both.