Battery
A Battery can be one cell or many cells.  Each cell has an anode, cathode and electrolyte.  The electrolyte is the main material inside the battery.  It is often a type of acid, and can be dangerous to touch.  The anode reacts with the electrolyte to produce electrons (this is the negative or - end).  The cathode reacts with the electrolyte and takes electrons (this is the positive or + end). An electric current happens when a wire connects the anode to the cathode, and the electrons move from one end to the other. (But a battery can be damaged by just a wire connecting the two ends, so a load is also needed between the two ends.  The load is something that slows the electrons down, and usually does something useful, like a lightbulb in a flashlight, or the electronics in a calculator).