A number from 0 through 1023 used to identify a network service on an IP network. Residing in a field in the TCP or UDP header, the port number directs packets to the appropriate application in the server. The most well-known port is 80, which identifies HTTP traffic for a Web server.
For a list see: Category:Ports
Note that well known ports are only those, that are in the range of 1 - 1024 and registered with the IANA, while there are also registered ports above 1024 and other unregistered ports, that may still be commonly used, but where software cannot rely on using them exclusive or complain on conflicts with other software.