BitTorrent itself is totally safe. There's no viruses or anything in it.
However as a program that allows you to download just about anything else you can think of... Sometimes those other things you get might not be safe. Torrent files are notorious for including viruses or malware... However, if you're savvy, and keep a keen eye on what torrents you pull, you can generally stay pretty safe.
There's more to it, but it basically boils down to knowing what type of file you're expecting to receive. For example, if you're looking for a movie, don't download a "movie" file with a name that ends .exe, that kind of thing. MP4, MKV, AVI, are some common legit movie types, and music will typically always be MP3 (might also be bundled with M3U as well, which is just a playlist file)