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.
Small Blocks Versus Big 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.
Crate Engine/Crate Motor
A crate engine or crate motor is an engine that can be bought in varying stages of readiness, typically considered short block, long block, or turn-key. These engines are typically purchased as either a replacement for a failed engine or an upgrade to replace an engine already in a vehicle. While most crate engines are nearly exact matches for what can be found in a vehicle that can be purchased as a whole, there are some engine specifications that are exclusively crate engines.
Short Blocks Versus Long Blocks
The terms "Short Block" and "Long Block" are typically used to describe an engine that is being sold or trasferred, so the recipient has a general idea of what they are getting.
A "short block" generally refers to an engine with minimal parts installed. In many cases, the defining line is if the cylinder heads are attached, with short blocks being the ones without the heads. On V, inline, and slant engines, the heads are typically above the highest point on the block, so having no cylinder heads on the block makes it appear shorter.
A long block will generally have cylinder heads, camshaft(s), timing components, and other vitals installed and ready to use, depending on the supplier.
Turn-Key Crate Engines
A turn-key crate engine is an engine package that is supposed to be the least work to get going, often coming with a wire harness and stand-alone ECU so that only a few basic electrical connections have to be made to get the new engine running and moving the vehicle.
Engine Block Halves
Some engines have block halves, like many of Subaru's boxer engines. These halves will be bolted together to form a singular block that can be disassembled for service.