Almost every desktop computer in existence now implements a protocol whereby numbered ports can be opened to the outside world. Most of the low-numbered ports have consistent assignments. For instance, port 80 is generally used to accept Web browser connections, and 25 and 110 are used for e-mail delivery and retrieval, respectively. Microsoft Windows does the bulk of its networking on ports 137, 138, and 139.
On Unix, look at the /etc/services file for an extensive list of network services and the port numbers associated with them.
A port is useless until it is associated with a socket, since a port is hardly anything more than a number, and a socket is a real IO object. Sockets are the means by which your Ruby scripts talk to ...