Understanding the two-sided nature of the web is important to understanding how a web site works. Make sure that you understand, at least in a general way, that there are several ways to extend the basic functionality of the server-client environment. HTML is the basic language of the web, both the server and the client understand it. But, it is a small and limited language. Many functions that you commonly see on the web are not part of HTML, but are extensions to either the server or the client.
Rollovers are an example.
Remember that the HTTP stands for Hypertalk Transmission Protocol . You must know how to build and manage directory structures on the web server. It is essential that you understand how to write paths to files and services your web site uses. All web sites uses basic tree structure. It's all about the user.