The HTML page is the basic molecule of the Web. It is the basic package for content on the Web, since Web sites are simply (admittedly a gross oversimplification) collections of Web pages. HTML pages provide the text for pages, along with pointers to images and other content, along with the instructions for how to put it all together in a pleasing display.

• An HTML file is a simple text file, where annotations called HTML (HyperText Markup Language) have been embedded in the text. No special editor is required for working with HTML, although specialized editors can be a big help.
• HTML provides a way to describe both the semantics (what it "means") and the presentation (how it looks) of the information on a Web page.
• HTML also provides (hypertext) linking, a mechanism for describing relationships between things.
• Cascading Style Sheets (CSS) enhance HTML by providing richer ways to describe the appearance, meaning, and behavior of elements in an HTML page.
• Javascript (JS) is a programming language that runs inside Web browsers. Javascript programs can directly (and asynchronously) interact with Web pages and Web servers. Javascript is typically used to display menus and image rollovers, and it is the core technology behind Ajax.

Speaking metaphorically, think of a Web site as a house. A Web page is a room with the house, with its own walls, floors, ceilings, plumbing, wiring, etc. A CSS style sheet is the interior designer's plan for how it will look (what the wall looks like, not that there is a wall). A designer has a master plan for the house, as well as a specific plan for each room; the same is true of a style sheet. Javascript is what happens when the electricity gets turned on, and things start happening (lights and fans come on, etc).