One of the most popular communication application in computers is chatting, or realtime-communication. It is started from text based "instant" messaging. As technology evolutions, this communication application grows up. Computer goes faster and faster, internet broadband is wider and still goes wider.
The information is goes bigger, from text to voice conversation. Real time! Skype is the most popular for voice communications over the internet. Yahoo Messenger and Google Talk also sports voice communication. And, today, video communication is a standard feature for communication applications within the internet.
You can use some kind of digital video cameras as webcam. But, it's so terrible. Most of them are big! It's become a problem when you use a laptop computers. The cams is bigger than you computers. Hehehe...
Then, you can use webcams, that created specially for video communications, with a compact size. Today's laptop and some desktop computers is equipped with built-in webcam. For example, Apple built iSight in every their Macs, except MacMini and MacPro.
But, if your computer doesn't sports integrated camera, you can go for the webcams. The webcams, even made for Skype, most of them are can be use within another application such as Yahoo Messenger, iChat AV, and (maybe) PhotoBooth.
If today video communications are standard feature, what will be happens in tomorrow? The cost will goes cheaper by the time. Computer's power is doubled every one and a half year, due to Moore's Law. The bandwidth goes wider. Video conference is real now. Will holographic communication in some years in the future?