Firearms. They can be used with any type of firearm such as Pistols, Shotguns, and Laser Pistols. They are interchangeable with Bullets which are also compatible with all firearms. In addition to allowing you to fire your gun, they also add a small amount of damage. Cells bought from a vendor will usually be 1d2 or 1d3, but higher-quality Cells can go as high as 2d8+X.
Equipping a cell gives you unlimited normal ammo, although Special Ammunition requires reloading at a Weapons Vendor or a Special ammos box for everything except Timestop.