Wireless guitars that were made for Rock Band 3's Pro Mode. Works basically the same as a real guitar and can be plugged into an amp. The way it works is that there are sensors inside the fret board to detect which fret your fingers are pressing on.
Comes in two forms, a Squier Stratocaster, and a Fender Mustang.
If you know how to play real guitar then you shouldn't have too much trouble with it, 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 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 and won't be recognized by Rock Band 3 and some may penalize you by detecting another input. It's also just better overall.