which css class takes effect?

[Charlie123]

Hi gurus,
This code snipet:
div#top_menu_block /* 1st */
{
position:absolute;
top:-23px;
left:6px;
height:23px;
border:0px solid red;
}
div#top_menu_block ul /* 2nd */
{
position:relative;
margin-left:auto;
margin-right:auto;
margin-top:0px;
margin-bottom:0px;
padding:0px;
width:auto;

}
div#top_menu_block ul li /*3rd */
{
position:relative;
list-style:none;
float:left;
width:108px;
height:23px;
}
I don't understand the meaning of 3 top_menu_block.
1: What does each of the top_menu_block mean? ( I am not asking the meaning of the inside contents).
2: Is the 1st one for div element, the 2nd for ul element in div but not for any div element?
3: If a ul element is inside a div element, which( 2nd or the 3rd ) will takes effect?
Thanks alot.
CHarlie