Skype is a VoIP service community website, is like a voice call version of instant messenger. You need to register an account with Skype and download a Skype software (which act like a software phone) to make call to your friends on Skype network.
I appreciate what Skype has develop with their software, the sound quality over Internet is good and it is available on all major platform and operating system including Windows, Mac OS X, Linux, Windows Mobile, iPhone, Android and some of the Nokia phones. The missing platform is Palm (Palm OS and webOS).
I have tried Skype on HTC Touch Windows Mobile platform some time ago, the sound quality is smooth while running on a 400Mhz CPU, even though other running programs become not responsive while I talking through the Skype software.
A lot of software is not available on multi-platform, even there is a need. A good example will Nokia PC Suite. The Nokia OVI Suite is still not available on Mac OS X and Linux, after heard that is "coming soon" for years.
Skype is slowing down these few years. I used to have about 20 friends on Skype network but only left with 1 or 2 will be online. Not everyone need voice call all the time, most people will prefer texting.
In terms of software development, Skype still on the right track and doing well.