These elements are a wrap around to #page and #page-wide. By default they do not actually contain anything inside it.

However there are some cool things you can do with it, your #page and #page-wide contain the static width of either 780px or 980px and say you want to extend the outside of that area by a few pixels and put something around the outside this is what you would use.

For example:

#page-wide-wrap {
       padding: 0px 10px;
       background: #ccc;
}

This will make it so that there is a 10px outline on the left and right of the static width #page-wide area. You can put a background there if you like, some people have put a background that repeat-y’s down the side of the #page-wide like a drop shadow or some funkey design.