What the heck are big blocks and small blocks?

When hearing people discuss older engines, you'll often hear them talking about a "400 big block" or "350 small block." The block is the main, bulky part of the engine (the cylinders are part of it). The adjectives "big" or "small" just refer to the size of the block. The number refers to of the volume (in cubic inches) of the cylinders. An engine can have the same horsepower and be big or small block (however, big blocks are heavier and small blocks are more limited in high horsepower).