Skype is an instant messaging program that happens to have a peer-to-peer (modeled after Kazaa) global voice network at its disposal, so you can use it to call people on your buddy list using your PC or Mac. All you need is broadband, a microphone, and a pair speakers or headphones. Voice calling alone doesn't set Skype apart from other IM applications like AIM or Windows Messenger--they also support voice. But Skype supports voice calling in a way that those applications can only dream of: Skype works in almost any broadband-connected network environment, even networks with firewalls that often break other voice-chatting apps. Plus, Skype's variable-bitrate sound codec makes it less prone to sound quality issues than its predecessors. In a nutshell, Skype just works. Perhaps that's why Skype's official slogan is "Internet Telephony that Just Works."
The world has noticed. 150 million downloads later, Skype now offers the ability for its users to call regular phone numbers from their PCs, a feature known as SkypeOut. Skype also offers a voicemail service and can route incoming calls to a certain phone number right to a user's desktop PC. There's even a Skype API that allows Windows and Mac programmers to integrate the Skype client with other applications. Videoconferencing add-ons, Outlook integration, and personal answering machines are just some of the cool software folks have developed using the Skype API.
But Skype can't take all of the credit for the recent growth of Voice over IP. A number of enterprise telephone system vendors have heavily promoted what they call "IP telephony"--the art of building corporate phone systems using Ethernet devices and host-based servers instead of old-fashioned PBX chassis and legacy technology. Cisco Systems and Avaya were two of the earliest players in the VoIP-phone-system arena, and their stubborn support of IP-based voice technology is beginning to pay off. More and more corporate customers are integrating IP phones and servers, and upgrading their IP networks to support voice applications, interested primarily in the productivity boost and long-term cost savings of running a single converged network instead of maintaining legacy voice equipment. This transition is a lot like the move from mainframes and minicomputers to personal computers a generation ago.
On two fronts--the corporate phone system and that of the home user--VoIP is transforming the global communications matrix. Instead of two separate notions of a global network (one for voice calling and one for Internet Protocol), a single converged network is arising, carrying both voice and data with the same networking protocol, IP. Steadily, corporations and domestic phone subscribers are migrating their voice services from the old voice plane to the new one, and next-generation, IP-based phone companies have rushed in to help them make the move.