External style sheet is generally a separate file that is used to declare all the styles that one want to use on their website. Then you can link to the external style sheet from all the HTML pages.
An external style sheet can be linked to an HTML document using the HTML’s link element-<LINK REL=StyleSheet HREF="style.css" TYPE="text/css" MEDIA=screen>
Here, the <link> tag takes the optional media attribute that specifies the media to which the style sheet need to be applied.