The services provided by professionals in particular field. For example, a company may charge a certain sum of money for the certain tasks their consultant accomplished for the company who ordered the service.
Skype differs from other VoIP-systems by several factors:
Skype is very popular. During the first week in August of 2003, more than 60 thousand people downloaded the Skype-client. Today, Skype is available for the following operating systems: MS Windows, MacOS, IoS, PocketPC, Android and Linux.