Gadgets are a group of technology in Ratchet & Clank that are almost always non-lethal. They differ from Items in that they are usually able to be used in a variety of different scenarios, and aren't equipped permanently. For instance, the Swingshot is equippable and is only used in specific segments of the game, whereas the Bolt Grabber Item is equipped permanently and functions indefinitely. This page talks about the gadgets in Ratchet & Clank, and also provides an appropriate list of them.
There are four groups of Gadgets in the original game: Hand Gadgets, Backpacks, Head Gear and Foot Wear. While all the Gadgets in every group are able to be equipped manually (and thus for stylistic purposes outside of their normal usage), only Gadgets in the Foot Wear category will equip automatically when needed (however the O2 Mask will also do this), which spares the user the trouble of selecting the Gadget manually. Only Hand Gadgets can be placed into the Quick Select menu.