Welcome to http://www.btfaq.com!
If you are new to BitTorrent, this site should help clear up a lot of commonly asked questions.
Please note: This site is mostly a mirror of the FAQ at http://dessent.net/btfaq/. That site is updated first, so this one may be a little bit behind in some places.
If you would like to check your firewall/NAT port forwarding configuration, try the Natcheck page.
Disclaimer: I am a Windows user, and so by default the answers will tend to apply to Windows if not otherwise stated. I’ve tried to include as much Linux and Mac information as I can. If you have anything to add on those subjects, by all means please contribute.