What is the difference between web app and website?

The dividing line is state. A website serves the same content to everyone and can be cached aggressively; a web app knows who you are, stores what you did, and has to be correct when two people change the same thing at once. That single difference brings in accounts, permissions, a database, and a much larger surface to test.

It also changes what happens after launch. A brochure site can sit untouched for a year; an application has users depending on it, which means monitoring, backups you have actually restored from, and someone available when it breaks. If you are describing logins and saved data, budget for the running of it and not only the building of it — the ongoing cost is the part that surprises people.