<link> is used to show a relationship between one document and another. For example, most people use <link> to include CSS styling written in another document in the HTML document they are displaying (the web page). It's done like this:
<link rel="stylesheet" href="style.css" type="text/css">
rel is stating what the
relationship with this html document the other page is. It's a stylesheet.
href is saying where the other document is.
type is stating how the other document is presented, in plain text with css syntax and data.
<link>