VOIP is an acronym which stands for "Voice Over IP".Most of us are familiar with the "Public Switched Telephone System” (PSTN), which allows us to contact people around the globe by dialling a sequence of numbers. VOIP offers an alternative, which works by routing digitised voice signals over IP networks, such as Company Intranets, or in some cases the public Internet.
On the face of it, the PSTN hasn't really changed much in more than 100 years. There have been many technology changes and improvements, such as tone dialling and Caller ID, but as far as the user is concerned, it's still a matter of dialling (more recently, pressing) a sequence of numbers, and getting connected to the person who's number was dialled. However, what happens behind the scenes to make this happen has changed considerably in recent years.
VOIP isn't a particularly new technology; there are papers and patents about the subject dating back several decades, and there was some early VOIP software available as early as 1991. The basic principle is pretty simple; it is essentially the same technology that is used to stream music across the internet. Voice sounds are picked up by a microphone and digitised by the sound card. The digitised audio is then compressed using an audio codec. This works by removing unneeded data, while maintaining the legibility of the audio, to make the stream compact enough to be sent in real time over the network.