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Cascading Style Sheets, or CSS, allow you to specify things like the font you want on your page, the size of your text, whether the page is to have 2 columns, whether your text is to be in bold or italics, and so on. In other words, it is the part that lets you control the appearance of your web page.
You may be used to the Microsoft Word "doc" format, where everything from the text you type to the appearance of the document is specified in a single file, transparent to you. On the web, the raw information is specified in HTML and most of the appearance is determined by the CSS.