What is a port?
A TCP or UDP port is what is used by an internet server to distinguish between requests for different services. For instance, traditionally telnet is served at port 23, while a web server listens for requests on TCP port 80. Ports are sometimes shown at the end of a URL; for instance the url telnet://server.com:9900 refers to a telnet server that is running on port 9900 of the host server.com.