salve a tutti io ho scaricato questo css da internet

codice:
<style type="text/css"> #pad {height:140px;} /* ================================================================ This copyright notice must be untouched at all times. The original version of this stylesheet and the associated (x)html is available at http://www.cssmenus.co.uk Copyright (c) 2009- Stu Nicholls. All rights reserved. This stylesheet and the associated (x)html may be modified in any way to fit your requirements. =================================================================== */ #menu {padding:0; margin:0; list-style:none;}
 #menu li {float:left; margin-left:1px; padding-top:2px;} 
#menu li a {display:block; height:38px; line-height:38px; padding:0 20px 0 0; float:left; background: url(tab_a.gif) no-repeat right top; color:#000; text-decoration:none;} 
#menu li a b {display:block; height:38px; float:left; padding:0 0 0 20px; background: url(tab_b.gif) left top; cursor:pointer;} 
#menu li a:hover, 
.home #menu li#home a, 
.single #menu li#single a, 
.dropdown #menu li#dropdown a,
 .dropline #menu li#dropline a,
 .flyout #menu li#flyout a,
 .support #menu li#support a,
 .contact #menu li#contact a 
{background-position:right -51px; margin-top:-2px; height:40px;} 
#menu li a:hover b, 
.home #menu li#home a b, 
.single #menu li#single a b,
 .dropdown #menu li#dropdown a b,
 .dropline #menu li#dropline a b, 
.flyout #menu li#flyout a b, 
.support #menu li#support a b,
 .contact #menu li#contact a b {background-position:left -51px; height:40px;}
 </style>
la parte finale nn la capisco praticanmente da dvoe iniziano le class nn riesco a capire cosa vogliono dire qualcuno mi può aiutare?