A Design Fragment is any one of the standard fragments which has had it's appearance altered using a CSS class which is specific to your selected design. This difference in appearance can be subtle or dramatic depending on your needs.
This piece of text is a Normal Text fragment which is being display using the ProductNews class. The designer of this site has decided to call it a 'Product News' fragment.
The text on the right hand side of this page is a Design Fragment. This particular fragment has been named a 'Product News' fragment.
The image below shows this fragment being editied, and because the design is based on a Normal Text fragment, it appears identical to the a Normal Text fragment apart from the name of the fragment being changed to Product News.