Where port numbers are concerned, consider an analogy between the computer and the telephone. Port numbers and their associated network addresses work something like telephone numbers and associated network extensions, or channels.

In this analogy a port number represents the "channel" for network communications. Port numbers allow different applications on the same computer to utilize network resources without interfering with each other. Just as different organizations may use different extension numbers "inside" their primary phone number, different computers use the same set of port numbers.
Port numbers most commonly appear in network programming, particularly socket programming. Sometimes, though, port numbers are made visible to the casual user. Normally, a Web site uses port number 80 and this number need not be included with the URL (although it can be).
In IP networking, port numbers can theoretically range from 0 to 65535. Most popular network applications, though, use port numbers at the low end of the range (such as 80 for HTTP). The port number is included as a field within the header of each IP packet.


There is a lot of information on port numbers: search google for "well known port numbers"
For a complete up to date list of port numbers see http://www.iana.org/assignments/port-numbers