When you kill a mob, every mob included in the game, the items it dropps are only able to be picked up by you for 20 seconds. When this timer runs out, everyone else will be able to pick up the items. You can also drop an item, and that item will only be able to be picked up by you for 20 seconds as well (Default 'Q' key.).
There is also an Item Guidewhich shows the stats of all items, and the approximate Identifications for each item.