Like Final Fantasy XIII, extras are unlockables you can use out-of-game when you complete certain objectives. They're always tied to major achievements, like completing the storyline or non-trivial achievements (gold or platinum trophies in the PS3 version).

• For the PS3 version, the extras available are XMB themes.
• For the XBox 360 version, the extras available are gamer pictures/icons.

Like Final Fantasy XIII, extras are unlockables you can use out-of-game when you complete certain objectives. They're always tied to major achievements, like completing the storyline or non-trivial achievements (gold or platinum trophies in the PS3 version)¹.

• For the PS3 version, the extras available are XMB themes.
• For the XBox 360 version, the extras available are gamer pictures/icons.

^{Note 1}: GameFAQs has the specific info about how to unlock them.