A port can be thought of as a channel of communications to a machine. Packets of information coming into a machine are addressed not only to that machine, but to that machine on a specified port. You can think of a port as a radio channel if you like, but the fundamental difference between a radio receiver and a computer, is that the computer can listen to any / all of 65000 possible channels at once!

However, the thing to note here is that typically the computer is not listening on very many ports at all. The computer will not respond to data or connection requests that come in on a port that it is not listening to.

One other thing to note, is that there are a number of important predefined ports which are universally used for various services. The major ones of these are: