For this achievement you will need to collect every type of item in the game, there are 380 total items, you can keep track of what items you have in your logs in the menu.
Visit this link for the full list of items:http://www.xbox360achievements.org/forum/showthread.php?t=37...
Thanks to Murbay for this awesome list.
Be sure when you look at the list to read the ones with red text as these are MISSABLE items, and if missed you will not be able to go back for them, thus forfeiting this achievement.
Note: Item No. 317, The Eyepatch is also missable, be sure and talk to the Traveler in the Lal Mountain Sheep Tribe Camp before heading off to the Great Desert (where you will find the Handwritten Manuscript you must trade him.) If you do not the traveler will vanish from the village and never trade for the eye patch. This occurs on Disc 3.
Thanks to albajos for the tip.