Technically, there is something which is called the 7 layers of the OSI model. I am not covering this, because I believe it is far too theoretical. However, there is one real good message of the OSI model and this is that each layer is built on top of the lower layer, but technically independent form that layer.

What does this mean in practice? There are several "real-world" meanings, one of them being that there is the Internet itself, then there are applications that are built on top of the technical capabilities of the Internet and then again, there are applications that are built on top of the Web which itself is an Internet application.

So, what are the first applications that were built on top of the Internet?

• One of the first Internet applications was E-Mail. E-Mail was created by two engineers who looked for a better way to exchange source code electronically. Today there are much better ways to do this, but back then it was a major breakthrough and in fact one of the "killer applications" that helped the Internet gain publicity. My favorite E-Mail client is Apple Mail, followed by Thunderbird.
• Another early application was Telnet. Telnet is used on Linux and UNIX computers (including Mac OS X) to remote login to a computer via a text-based console. Nowadays, we are typically using "ssh" (secure shell) which is basically a secure version of Telnet. Telnet and ssh are also available for Windows PCs.
• Then came FTP (File Transfer Protocol). FTP allows to transfer files between an FTP client (the original, text-based application is simply called "ftp") and an FTP server. Today there are tons of FTP client applications out there for all major operating systems. The one I like best on the Mac is "Transmit".
• And finally, there was the World Wide Web (WWW). The "Web" was originally created by Tim Berners-Lee at the CERN, a major research institution based in Switzerland. The Web really made the Internet accessible to the "rest of us" for the very first time. The first Web browser that was widely used was Netscape, developed by Marc Andreesen. What not all people know is that Netscape provides still the original, technical foundation for one of the most widely-used browsers out there, Firefox. I, being a Mac user, like Safari best, followed by Firefox. However, Google is going to introduce quite a few interesting concepts with "Chrome", so let's see...

Only much, much later came the likes of Yahoo Messenger, Skype and BitTorrent - which are also Internet applications, not Web applications, i.e., they directly make us of the Internet protocol stack (TCP/IP). Twitter, on the contrary, is a Web application meaning that is is built on Web technology, rather than on the Internet directly. We will also take a closer look at all these widely used applications, but not today. Have a nice evening ;-).