Web applications are a client/server-based model.
"Huh?" you ask.
Let's look at how these things work together.
When you request -- or want to visit -- a Web page, an HTML document or file is sent from that page's server down to your computer. Many times, that HTML document or file is going to be produced when you requested it thanks to a programming language on the server.
That HTML is probably going to contain CSS, which is used to make the Web page you want to visit look nice. CSS makes it easier to change the look of the HTML document for tasks such as printing and mobile phone display.
When you click a button or fill out a form on the Web page you've visited, the data you've provided are sent back to the server and complete some interaction with the Web application.
One key thing to know about Web applications is that they are stateless -- meaning they don't know one user from the next. What allows Web applications to identify individual users are known as cookies, which are bits of data that your browser sends to a server every time you request to visit a Web page. Cookies are limited by the domain name of the Web application, so cookies for Google.com are not sent to anyone other than Google.com. Without cookies, Web applications would not effectively be able to know you from anyone else accessing the application. Your click or form data are then processed on the server by one the programming languages, which, in turn, updates data in a database.