Ports

From AzureusWiki

You normally only have one physical connection to a network at a time, but often use it for more than one thing, or logical connection, at a time. Ports are used to tell the operating system which logical connection the packets of data it receives are intended for.

For example, Hyper Text Transfer Protocol (HTTP) usually uses port 80 while File Transfer Protocol (FTP) normally uses port 21. When you request a webpage, your computer sends a request packet to port 80 on the server, then the server knows to use HTTP, and not FTP. Port numbers run from 0 to 65535. Please see PortIsBlacklisted if you need information on which ports to use with BitTorrent and Azureus. You can go to http://ports.tantalo.net/ and http://en.wikipedia.org/wiki/List_of_well_known_ports_(computing) to find out more about which ports normally belong to which protocol/program.

Read all about port forwarding.

Read the Azureus FAQ