Pistons are smaller than the bore they fit into in the engine block. This allows for expansion as the engine warms and the pistons expand in their bores. Piston rings are used to fill this changing gap and contain the compression and expansion gases in the cylinder. Most automotive piston ring sets contain two compression rings and one oil control ring. The oil control ring contains three pieces; a spacer and two scraper rings that regulate a thin film of oil that lubricates the cylinder wall.
The rings fit into grooves machined into the pistons. The two top grooves are dedicated to the compression rings and the bottom groove dedicated to the oil control rings. Clean the pistons in a cold tank and clean these grooves with a ring groove cleaner before measuring. Use a new ring and a feeler gauge to obtain the measurements. The thickness of this groove is important. The compression ring grooves allow just enough compression to force the ring both downward and outward. This creates the tight seal needed to contain the compression and prevent blowby.
Compression rings are usually designed with a taper or a barrel shape. Because of these unique shapes, they must be placed into the piston facing in the right direction. These are typically designated with a dot on the top of the compression ring that must face upward toward the dome or top of the piston. Check for manufacturer's specifications.
They are made of steel, coated with molybdenum (Moly Rings) or chrome. Cast iron rings are often used when rebuilding an older engine and chrome rings are used for their durability. Chrome rings are known to wear the cylinder walls, so they're often used in heavy duty and off road applications, where dirt may enter the intake and scuff the rings.