You would have recorded your voice in your computer and played it later. In this process the sound is sampled at a very high rate and stored. In Voip (Voice over IP), the same is done and the difference is that the sound is sent across to another computer using the IP network and then played in the other computer. The technology that is used in place is the Voip technology. To send voice to the other computer, you need to compress it in your end and the same should be decompressed at the other end to save bandwidth. It will a lot of time if send it as it is. For this purpose Codecs are used for compressing and decompressing.