DTW files are a proprietary format created by Morgan Wireless for use in their "MyBetterDesktop" applications. These files are very similar to a normal background except that they may contain contain multiple pieces of content which it can display in different ways in order to spruce up the desktop through the user of non-static content.

Further, these files are completely self-contained. If you share a DTW file, the other user will have exactly the same content.