PeerBlock lets you control who your computer "talks to" on the Internet. By selecting appropriate lists of "known bad" computers, you can block communication with advertising or spyware oriented servers, computers monitoring your p2p activities, computers which have been "hacked", even entire countries! PeerBlock blocks "known bad" computers from accessing yours, and vice versa.
Depending on the lists you have it set up to use, you can block governments, corporations, machines flagged for anti-p2p activites, even entire countries! Whether you're sharing files with Bittorrent or just surfing the web, PeerBlock can help protect you from the bad guys. They can't get in to your computer, and your computer won't try to send them anything either.
PeerBlock monitors each connection your computer tries to make with another one - and each connection other computers try to make with you - and then compares the other computer's IP address against a list of "known bad" IP addresses to see whether it should let this connection go through or not.