The biggest and heaviest single part of the engine, the engine block is the piece one starts with when putting an engine together. A lot of parts are bolted right to the block (both inside and outside), often including the mounts, which connect the engine to the chassis. It is often a majority of what the visible lower half of the engine is. Usually, due to part and labor costs, if an engine block gets damaged, the entire engine (or even vehicle) is scrapped.
Differences Between Big & Small Blocks
These names are usually used to describe an auto manufacturer's two engine models, usually the V8s of American manufacturers. The cubic inches/liters of displacement aren't the refining factor, because a big block can have various bore sizes which get smaller than the biggest available in the small block's range.
Engine Block Halves
Some engines have block halves, like many of Subaru's boxer engines.