The wrap is essentially just a container. Keeps things nicely in a box.

Sometimes it's useful to nest divs, sometimes its not. For example, if you have a page where you want all the content centered, you'd have #wrap with "padding: 0 auto;".
Here's a site and its css file that uses some floats. Might help you, might confuse you. If you'd like I could probably find a simpler example:
http://beartoothkawasaki.com/
http://beartoothkawasaki.com/style.css
Note in the css:
Code:
#navigation {
width: 171px;
position: relative;
float: left;
}
#content {
width: 601px;
position: relative;
float: left;
}