Skype maintains its registration servers but doesn't maintain any packet servers that it can use to collect and distribute user VOIP packets. Skype converts some users computers, unknown to them, into proxy servers by establishing what it calls "SuperNodes" on certain users. Those users agree to allow this to happen on their computers because they clicked through the user agreement, even though most do not read it and would not understand it if they did read it.
A computer selected to launch a SuperNode is usually one with a fast CPU, several GB of RAM AND a cable Internet connection. When my PC was selected as a SuperNode it took up to 340Kb/s of my bandwidth! It was a noticeable slowdown, sometimes causing the mouse to hesitate during startups of certain other applications. Also, when the SuperNode was running and I was connected to someone with Skype, the video began to lag to audio by up to 20