External CSS files are plain text, just like HTML, except the file names end in .css. CSS files ONLY contain CSS rules. No HTML. Linking to CSS files is easy too. Its done using two methods, both of which get placed in the HEAD of your HTML document.
<link rel="stylesheet" type="text/css" media="screen" href="url/to/stylesheet.css">
The above method allows a style sheet to be downloaded by 4.0 and newer browsers. Netscape 4.x has a quirk where it ignores the above tag completely because of the media attribute. If you want a style sheet to be read by ALL 4.0 and newer browsers, use the above HTML tag and omit the media attribute and value.
The second method is using the @import rule.
<style type="text/css" media="screen">
Browsers 5.0 and newer will download the style sheet using the @import method. Using a combination of LINK and @import style sheets, you can feed simple style sheets to 4.x browsers and more complex style sheets to 5+ browsers. Browsers 5.0 and newer will read the LINK style sheet and the @import style sheet. Advanced CSS 1.0 and 2.0 styles can be put in the @import style sheet, while commonly supported CSS 1.0 styles can be placed in the LINK stylesheet.