DTS (Digital Theater Systems) is a multi-channel surround sound format that is often found in movie soundtracks and is used for multi-channel sound on film, DVD, laser discs, and even some CDDAs. It is similar to Dolby System's AC3 format (it's primary competitor), and they are similar in terms of audio performance. More about the format can be found on Wikipedia's DTS page.
DTS wave files are automatically decoded and output as 5.1 using Media Center 16 or 17. You can use .wav or .dts as the extension.
Version of Media Center before v16 may not nicely support DTS WAV files.
There is currently no easy way to bitstream DTS (or any other audio format other than DSD) using Media Center. In general, it is better to decode on the computer anyway. Media Center has the highest possible quality decoder, and decoding with Media Center allows other operations like Room Correction, volume, and more.
(For DTS files converted to WM Lossless, please see this thread on Interact.)