Some files are posted as "ISO" files. You can not simply play them in a player, they must either be burned or mounted to watch.
To burn it you could use Nero or another free burning utility such as CDBurnerXP Pro:
http://www.cdburnerxp.se/Mounting:
http://en.wikipedia.org/wiki/Disk_image_emulatorDaemon tools:
http://www.daemon-tools.ccAfter mounting you can watch it like a real DVD, use a player on your computer to access the \"mounted\" drive as you would when viewing a real DVD.
Some torrents come to you as a directory called \"video_ts\" with files such as .ifo and .vob, these files can be burned with some burning programs only. The program has to support a \"DVD Video\" option, when you find that option, you drag and drop all of the files inside of the VIDEO_TS folder to the VIDEO_TS folder on the DVD and then burn.
To watch the VIDEO_TS files from your hard drive, drag and drop the folder itself to a video player. This method only works on some players.
Mac:
http://www.kenstone.net/fcp_homepage/making_dvd_copies.html