Basics of FaceTime in OS X Mavericks
In the beginning, FaceTime brought video calling to the iPhone 4. Not surprisingly, it soon spread to OSX Mavericks, later generation iPads, and the iPod touch.
One of Messages features is video chat. Alas, Messages can only video chat with folks on Macs or PCs; FaceTime lets you do it with other Mac users as well as users of iPhone 4s, iPads, and iPod touches.
In addition to its aforementioned video-with-iDevices prowess, FaceTime works very nicely for Mac-to-Mac video calls. And because it's a single-purpose application, many users find it easier and less intimidating to set up and use than Messages or Skype.
By the way, there's no Windows version of FaceTime at the moment, so you'll have to use Messages (or third-party software like Skype) to have cross-platform video chats.
To get started, just launch FaceTime from either your Applications folder, your Launchpad, or your Dock, and the main (only) FaceTime window appears.
The left side of the window shows what your Mac's camera is seeing.
Click the iPhone entry to initiate a call. You can FaceTime with more than one person if they are together.
FaceTime uses Mavericks' Contacts, so if you have friends or family who have an iPhone 4 or later, iPad 2 or later, iPod touch (4th generation or later), or a Mac, just click their phone number or e-mail address to initiate a video call