Guitars that were made for Rock Band 3's Pro Mode.
Comes in two forms, a Fender Mustang, and a Squier Stratocaster.
The Squier is just a normal Squier Straocaster that has been modded to include buttons to navigate the menus, and sensors inside the fretboard to detect which string you're pressing down and on what fret. It can be used as both a controller for Rock Band 3 and as a real guitar since it can be plugged into an amp and uses actual strings.
The Fender Mustang is ONLY for Rock Band 3 as all the "strings" are just buttons. It CAN however be used on a computer as a real guitar if you have a midi adapter.
If you know how to play real guitar then you shouldn't have too much trouble with either, but your playing has to be fairly accurate otherwise you may drop notes.
If you want to learn how to play actual guitar, there is a tutorial for how to use it, but it's recommended to just use Rocksmith since some techniques that you would use on a real guitar (bending, palm muting, harmonics, etc.) can't be used even on the Squier since they won't be recognized by Rock Band 3, and some may penalize you by detecting another input. It's also just better overall.
Only recommended if you want to try out the Pro Mode and already know how to play guitar.
Obviously neither can be used on Clone Hero since the Squier is just a real guitar with some mods, and while you may be able to map the buttons on on the Mustang, you'll probably just be limited to taps while the other 97 buttons do nothing.