There are a variety of collectibles scattered throughout the game that can be found in different ways. Some of these need to be found by looking in certain types of boxes, others are cut from the limbs of fallen enemies while others still are simply lying around on the ground waiting to be found. All of these are detailed within.
Note that Titles are the sole exception to these. They require you to complete certain objectives throughout an entire playthrough and only unlock once you've beaten the last chapter. They cannot be found as an item on an individual level. They are, however, listed in this section since they are listed as collectibles in the game menu.