The chain block contains two wheels or cogs in which the chain is wrapped around. Both electric chain blocks and manual chain blocks have the same fundamental functions - when the chain is pulled, it wraps around the wheels which adds torque to the lifting mechanism, pulling the item that is on the other end of the chain (usually with a strong hook) and lifting it off the ground - the wheels inside the block are how they are able to lift heavy loads - even when operated manually.
A chain block has a strong chain for lifting objects and attaching hooks to, manual lifting blocks have a hand chain which can be used for pulling by hand to bring the load up (these are powered by motors in electric chain blocks), they also have a grabbing hook for applying the lifting load. Electric chain blocks are the most common method, however, manual chain blocks are commonly used to lift lighter loads as they aren’t as expensive and are easier to move.