Yesterday, Apple [AAPL] released iOS 6 to the public. The new version builds on previous versions of the operating system. It is compatible with all iPhone models down to the 3GS as well as the latest 2 generations of iPod touch and the iPad 2 and iPad 3. For those running the latest gen iPhone 5 (released tomorrow), this OS makes use of the larger screen and also has all the apps adapted to fill the extra space on the screen.
Although iOS 6 is compatible all the way down to the iPhone 3GS, not all features are made available to all devices. Here is a quick run down of what you do and don't get with the new version of iOS.
To get the full experience you'll need either an iPhone 4S or iPhone 5. Both devices have all the features other than the stretched out apps (more screen estate) when comparing the 5 to the 4S. The iPhone 4 lacks several items which includes FaceTime over Cellular, Flyover (on Apple maps), Panorama, Turn-by-turn navigation and Siri. The biggest let down there is probably the turn-by-turn directions as many were looking out for a free option with the new Apple Maps.
As for the iPad, the new iPad gets everything but Panorama and Passbook and the iPad 2 lacks Siri and FaceTime over Cellular. Other than that, iOS 6 does bring a fair few decent new things to users.
In my honest opinion, iOS 6 feels more like an iOS 5.2 update rather than being numbered 6 other than for the latest generation models (and 4S) who get turn-by-turn navigation.
As for downloading the latest OS, if you are already on iOS 5, you should be able to just check for updates on your device and update through that way. Alternatively you can connect up to iTunes and update with a cable attached. Whatever option you choose, make sure you get a good backup before hand either by forcing one on iCloud on the device or by manually grabbing one in iTunes when the device is connected. The reason for this is that I have heard of some devices losing data when restored with the new iOS. If you have a backup then you can simply restore the backup after the update and have everything as it was before hand.