Battle Lou in Easy Mode
Description: Hint for when you battle with Lou using Easy Mode notes.
When the battle with Lou starts (on Expert Mode), strum without holding any frets down. When you lose, select the Tutorial option at the Game Over menu. Do the Guitar Battle Tutorial and play through it in its entirety. After the "Lesson Complete" message appears on-screen and the next fret board appears, pause the game and exit. This will bring you back to the Lessons menu.
From there, press the red button, which will bring you back to the list of songs. The battle with Lou should be highlighted. Choose the song, and Lou's note chart will be set to Easy Mode now. Your note chart will still be in Expert Mode, but Lou's will be in easy mode.

Guitarists
You can unlock Tom Morello (of Rage Against the Machine),
Slash (of Guns N Roses/Velvet Revolver),
Lou (the Devil in the game), and a character called MetalHead by beating them in the game and then purchasing them for $10,000 ***Lou costs $15,000.

Beach Life Bass
How to unlock: complete Co-op Mode on hard.

PendulAxe Bass
How to unlock: complete Co-op Mode on expert.

Through the Fire & Flames
To unlock the track "Through the Fire and Flames" you must beat the game.

Precision Mode
Description: Unlock Precision Mode
How to: Insert the following cheat code notes in the Options menu:
Green & Red x3, Red & Yellow x2, Red & Blue x2,
Yellow & Blue, Yellow & Orange, Yellow & Orange,
Green & Red x3, Red & Yellow x2, Red & Blue x2,
Yellow & Blue, Yellow & Orange x2.

Hyperspeed
How to unlock: head to the cheat menu that is found under Options and insert the following cheat on your guitar:
Orange, Blue, Orange, Yellow, Orange, Blue, Orange, Yellow

Nemesis 13 Guitar
How to unlock: complete Co-Op Career Mode on any difficulty.

Unlock Songs (Normal)
To unlock songs in the normal fashion, just pass the song successfully.

Saint George Guitar
How to unlock: complete Career Mode on Medium.

Rojimbo Guitar
How to unlock: complete Career Mode on Hard difficulty.

Distant Visitor Guitar
How to unlock: complete Career Mode on Expert difficulty.

Moon Shot Guitar
How to unlock: complete Career Mode on Easy.

Jolly Roger Guitar
How to unlock: get five stars on each song during Career Mode on Medium difficulty.

Bat Guitar
How to unlock: get 5 stars on each song during Career Mode on Easy difficulty.

Air Guitar
Description: unlock the Air Guitar
How to unlock: Insert the following cheat code notes in the Options menu:
Blue & Yellow, Green & Yellow x2, Red & Blue x2,
Red & Yellow x2, Blue & Yellow, Green & Yellow x2,
Red & Blue x2, Red & Yellow x2, Green & Yellow x2,
Red & Yellow x2

Neversoft Skateboard
How to unlock: complete Co-Op Mode on expert with 5 stars.

Tiki Guitar
How to unlock: complete Career Mode on hard with five stars.

Performance Mode
Description: Unlock Performance Mode
How to unlock: Insert the following notes in the Options menu:
Red & Yellow, Red & Blue, Red & Orange,
Red & Blue, Red & Yellow, Green & Blue,
Red & Yellow, Red & Blue

El Jefe Guitar
How to unlock: complete Career Mode on expert with five stars.

All Songs (Quick Play Mode)
Description: Unlock all of the game's songs for Quick Play Mode
How to unlock: Use the following notes in the Options menu:
Yellow & Orange, Red & Blue, Red & Orange,
Green & Blue, Red & Yellow, Yellow & Orange,
Red & Yellow, Red & Blue, Green & Yellow x2,
Yellow & Blue x2, Yellow & Orange x2,
Yellow & Blue, Yellow, Red, Red & Yellow,
Red, Yellow, Orange

Boss Characters
Description: Unlock any boss characters as playable character
How to unlock: Just beat them in the game and they will become unlocked.