Skype is a voice over IP software that allows you to make calls from computer to computer or have the computer call a cell phone or a LAN line phone.  Calling cell phones and LAN lines requires you to subscribe to the paid service. 

There are fantastic and very easy to use help guides that walk you through all aspects of using Skype:

http://www.skype.com/help/guides/

The guide for creating your own chat group is:

http://www.skype.com/help/guides/chats_windows/

Skype has many classroom uses:

Tech support network

Grade level, Team or New Teacher network

Connecting students with others globally.