why the quality of sound is better than MSN and Yahoo ?
Is a client/server application ?

it's not really an API question, but anyhow -- skype is a pure P2P application. the calls never get routed through any central servers. as for the sound quality, our core developers have worked real hard to get as good audio as possible into a data stream as thin as possible.

you may want to read www.skype.com/security/guide-for-network-admins.pdf for a bit more background information.