Skype is a software application that allows users to make telephone calls over the Internet for free between other Skype users, while calls to landlines and mobile phones can be made for a fee. Additional features of Skype include instant messaging, video conferencing, and file transfers. The document provides instructions on how to use Skype's instant messaging, audio calls, video calls, conference calls, and file transfer features. It also provides tips on adding contacts carefully and setting rules for conference calls.