If I can upload the other document I will, but here is an extract from some seperate documentation I have.
The Skype system adapts dynamically to the network environment it finds. As with KaZaa, Skype may conclude that the most effective way to route a communication is not directly between the called and calling endpoints, but instead to send the traffic via another Skype client not otherwise involved in the call. An intermediary client used in this way is known as a super-node. Super-nodes are not pre-defined or configured, but are a dynamic feature of the Skype client software. Any Skype client that discovers it is well connected to the Internet is likely to offer itself as a super-node by advertising its connectivity to other Skype users. As a result, a PC that has access to significant bandwidth and runs the Skype client software may handle voice communications to and from clients all over the world, not just those originating or destined for the local user of the PC. Networks with super-nodes may experience large flows of inbound and outbound traffic that have no connection with any local user. A user who installs Skype with the default configuration permits his computer and his organisation’s bandwidth to be used by any other Skype user.
A feature of Skype that may be very welcome to its users is that all communications are encrypted end-to-end between the two communicating clients. Skype effectively provides an encrypted tunnel through the firewall that could be used for attacks against the client PC and any other networked devices it can connect to in turn.
Skype also provides a mechanism for third party software to be written and installed within the system: the history of the equivalent function in web browsers suggests that this will be used at least as often by insecure and unwelcome software as by useful applications.
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