For each area there is a set list of items one can gather and pick up there when you go gathering. For each area there are a total of three different catagories for the items to go in when you put it into your collection.
Animals are, well, animals and plants are plants. The objects category is a bunch of random items that don't fit into Animals or Plants. These items include bones, stones, weapons, and wood items.
For the most part, every item can be used for something. Birds are equippable to your active feli, giving them feathers (some used also for patch markings), stones are now equippable to your active feli, giving them various effects and jewelry, and all wood can be used to expand your (or your clan's) coffers. There are even some plants, animals, and objects (mainly R-100 and some R-90) that can be used for patch markings as well.