A Virtual Toll Gate

A firewall works at your computer's ports. When we're talking about computer networking, a port isn't the same as a jack or socket you plug your monitor into. Rather it's a virtual entry point where your computer exchanges information with other networks. 

Every computer has lots of ports, each of which handles different kinds of data. For instance, according to the Internet Measurement Conference (IMC), emails often go to port 25 while webpages go to port 80 — even though they both come through the same internet connection