Web Application

There are many types of websites.  Some sites are simply built to display static content.  They don't require an underlying application system or database.

These simple sites consist of a static web pages, web parts, graphics, pictures, buttons etc. that can be sent by a web server to a web browser when a request for a web page is received.  Such sites are easy to build and inexpensive to operate.

World Wide Web - Conceptual Overview

Often, however, more complex website are needed.  For example, you might want to construct a website that computes a monthly amortization schedules based on input fields provided on a web form.

This requirement can't be met by a web server alone.  It requires program logic to take the form fields provided by a visitor (e.g. principal amount, interest rate, payment frequency, payment amount), calculate the amortization and display or print the schedule.

In this example, there is no need to authenticate users or store data in a database.  If there were such a need, the complexity of the website would be even greater.