Items found in game are key to success. You can carry up to three items in your backpack, but can also equip gear in one of your four gear slots. In other words, you can carry up to seven items at a time. There are three types of items: Consumable, Quest, and Gear.
Consumable items help you in certain ways when you activate them. For example, a med kit can heal you. Consumable items all have a set number of charges, and the item will disappear when you've used it up. Consumables can only be carried in your backpack, but you may stack consumables of the same type indefinitely.
Quest items are only found in certain levels and interact somehow with the level's mechanics. For example, the satellite remote triggers an ion strike on the laboratory level. Quest items can only be carried in your backpack.
Gear items can be equipped to provide a stat bonus. Your character has four gear slots: head, chest, hands, and feet. You can only equip gear of the corresponding type in each slot. Gear is also separated into five different quality tiers: idiot-proof, basic, upscale, supreme, and legendary. The required character level also ascends with gear quality, and so it's often more useful to pick up crappy gear at the beginning of the game.