I'm sure it's just simple html/css but I don't know what to call the bar (googling horizontal bar html always results in a horizontal rule).
http://jquery.com/ has one - the grayish bar the runs across the top separating the menu from the content of the page. I'd love to make one of my own.
There is a number of ways to do this.
On jquery.com it is part of background image applied the body tag.
You can have a header section which has background aligned to the bottom as an image and bottom padding that prevents text/content from overlaying that part. Finally you could use thick border if you want to just have plain color. I am sure there are numerous other ways to do this as well.