Normally, users download a client like uTorrent to download torrents.
jstorrent is a Chrome packaged app for downloading torrents. Although not completely stable, it does appear to be functional, and (as of writing) the only app based solution for downloading torrents.
You can of course run a uTorrent server on a remote machine and use the web interface on Chromebook to manage it, which is what this machine is best at. However, if you have no remote machine, you can install uTorrent server on the Chromebook itself, thanks to this document from François Beaufort.