The short version
A website presents information — hours, services, contact details, a menu. A web app does something — lets a user log in, manage data, place an order, track a delivery.
Why this distinction matters for cost
A website is usually a fixed, one-time build. A web app has ongoing complexity — user accounts, a database, security requirements — which means more development time and often ongoing maintenance.
A simple test
Ask: "does a visitor need to create an account or store data to use this?" If yes, you need a web app. If people just need to read about your business and contact you, a well-built website is enough — and costs a fraction as much.
Common mistake
Businesses often over-buy — commissioning a full web app with logins and dashboards when a clean, fast website with a contact form would have solved the actual problem. Start with what you need, not what sounds impressive.