Back when Skype was still a standalone company, users could specify their own usernames and customize it even after creating their account. A Skype ID is unique to each Skype account and identifies that account amongst the global collection of Skype accounts.
Hence, you can think of it in the same context that you think of usernames on other apps like Facebook, Twitter, and Instagram. Simply put, your Skype Live ID is your Skype username.
