"://" in URLs Explained

Recently, members of our message board have been discussing the syntax of URLs - specifically the use of :// characters such as in http://compnetworking.about.com/. While they are used by some network software to separate protocol from the host information, the choice of these particular characters made by engineers years ago was somewhat arbitrary. In fact, unless you are a programmer, you might never have to actually type these special characters anymore.
→ Discuss - URLs - Why "://" in URLs?