Skype is a software application that allows users to make VoIP calls. So, you can only use Skype from your computer. You can still call the public telephone network, but you'll need a webcam or a headset to use Skype to make a VoIP call.
Skype's residential service is most popular for its free user-to-user chat functions. But Skype also allows you to make and receive calls to traditional phones and other VoIP services with a debited account. Like most VoIP services, Skype also supports mobile applications. Once again, you'll have to pay for the off-network calls.
Skype has also expanded to include business services: Skype for SIP Open Beta and Skype for Asterisk. While both software solutions are free, using them is not. Both function on a credit-based system that Skype offers with residential service. While they work as a stop-gap measure, Skype's business services aren't optimized – yet. If Skype continues to build products with the same level of quality and ingenuity, it's only a matter of time until Skype rises to the top of the business VoIP market. But for know, you're better off with a more established VoIP service for your business or residential VoIP calling needs.