Wikipedia has a good page on it all here - should explain it all. In essence it is using the internet for telephone calls. Eventually *everything* will be "over the internet" - web pages like this, tv, voice calls, video calls, mobile phones, your electricity/gas/water meters.....
In 50 years the internet will be regarded in the same way as electricity and indoor plumbing, everything will be dependent on it. But as no-one "owns or controls" the internet, and it was designed from the ground up to be resilient to faults, damage, etc etc that's not a bad thing.
Sorry for my wafflng, it's a pet subject of mine.