The Fender Rockband is a gaming guitar with a built in string-to-MIDI converter. According to the description it sends MIDI, I think without pitchbend, so it can be used like a real MIDI guitar to a certain degree. But I wouldn't call it a MIDI guitar, it's a game controller.
For a standard MIDI guitar you need a MIDI pickup, a converter and a soundmodule (or software like Logic). Alternatively you can get a pickup plus a converter with a built-in synthesizer.
Pickup:
There are basically two types of MIDI pickups: magnetic or piezo. Magnetic pickups are mounted on regular guitars very close to the bridge. A controller with two buttons and a wheel is also mounted on the guitar. This is the classic Roland setup. Piezo pickups are mounted under each string, right in the bridge. Some guitars have such pickups already built-in (Godin for example) or you buy such a pickup bridge and mount it on your guitar.
Converter:
Converters transform the incoming signal from the strings to MIDI. They are slightly different in accuracy and speed. Speed means the time the converter needs to analyze the notes and transform them to MIDI information. Roland and Axon offer converters with and without soundboards.
You can turn any guitar into a MIDI guitar but not every pickup system fits on every guitar. In parallel to a MIDI pickup you can still play with your normal pickups and mix the guitar sound with MIDI sounds from the soundboard, an external sound module or your software.
MIDI guitars through standard converters send notes with velocity and pitchbend. Either all strings on one MIDI channel or each string on its own channel. Since the MIDI information is taken directly from the strings, the result is not like hitting a key on a keyboard. Good players can imitate keyboard- or other sounds but the instrument remains a guitar.
Of course there are also special (and very special) guitar and guitar-like MIDI controllers on the market.
Youtube has many interesting movies about MIDI guitars and converters.