Facetime is Apple's video chat program for all of its iOS devices and for modern Macs (OS X6.6 and above). It is installed automatically on iOS devices when you set up and activate it, and if it's not already on your Mac, it is available from the Apple Mac App Store.
Facetime's video quality is quite good, and the calls are free and work over both WiFi and cellular data. But it is limited doesn't provide for group video calls, file transfer, screensharing, VOIP telephony, and of course, you can only talk to someone else on a Mac, iPad or iPhone. Facetime requires a front-facing camera, so the original iPad and the older iPod Touch models without the chat camera can't use it.
Since iPads and Macs can also use Skype and Hangouts, Facetime's limitations aren't that limiting. Nevertheless, when I am video chatting from my iPad or iPhone with someone else on an Apple device, I usually choose Facetime for the video quality.