Skype is an online telephone, video chat, and instant messaging service that is used by hundreds of millions of people around the world. It is now owned by Microsoft and has replaced Microsoft Messenger as the company's communication platform.
With Skype, you can talk to other Skype users, anywhere in the world, by video, audio or instant message at no charge. You can also use Skype to call landlines or cellphones anywhere in the world, on either a pay-as-you-go basis (buy some credit and buy more if/when it runs out) or by subscription. For example, the US rate is 2.3¢ per minute to call landlines or cellphones, but you can have unlimited calling in US and Canada for $2.99 per month. If you're on WiFi at home, you can use Skype from your cell phone via WiFi and save your cell minutes.
Currently, Skype is available with a free downloadable app for almost every platform that has a microphone and a camera, including Windows, Mac, Linux, iPad/iPhone, Android, and Kindle Fire HD and HDX. However, it is not available for Chrome OS (Chromebooks and other Chrome platforms), nor does it work on Google TV.