It can be linked:
<link rel="stylesheet" type="text/css" href="css/ycsoftware.css" />
It can be embedded
<style> ... </style>
It can be imported
<style type="text/css" media="all"> <!-- @import url("css/ycsoftware.css"); --> </style>
You can also import css from within a css file
@import "jquery.ui.base.css";