Medium is the middle difficulty in Guitar Hero (hence the name). The difficulty uses only the green, red, yellow, and blue fret buttons.
Differences from Easy and Hard
Medium uses one more fret button than easy, which uses only the green, yellow, and red fret buttons. It also has more chords and hammer ons. It even has a few split chords in the more difficult songs. Medium is much less difficult than Hard, mainly because the orange fret button is introduced at Hard. Also, Hard has three note chords and more split chords. Medium also moves slightly faster than easy, and with more notes, some people find it easier to strum long strings of a note or a chord by simply strumming to the correct beat, whereas in easy one must follow a restricted note pattern. This is why some people say it is less awkward to play medium than easy.