When an engine has accumulated excessive miles or has had catastrophic damage, replacement with a new or re-manufactured engine can restore a vehicle's performance. Engines are available in various stages of completion: complete, long block assembly or short block assembly.

Assembly Stages

The terms "short block" and "long block" don't refer to physical dimensions but to the completeness of an engine. Each of the three assembly stages adds more parts, moving an engine closer to being ready to run. To better understand what a long block is, let's build up from the most basic assembly.

Short Block

Less complete than a long block, a short block is the most basic of engine sub-assemblies and consists of major components of the foundation of the engine, such as the cylinder block, crankshaft, connecting rods and pistons, camshaft, and timing gears and chain.

Long Block

A long block adds cylinder head assemblies to the short block. Long blocks still do not have all of the components and accessories necessary to be a running engine, but they are nearly complete.

Complete

Unlike a long block, complete engines are generally regarded as "turn-key" -- they have all the components needed to operate. All that is required is to install the engine and make the necessary electrical and fuel-system connections.

Other Parts

Most of the time, parts from an engine that is being replaced are still serviceable and can be transferred to a long block. These typically include the oil pan, timing and valve covers, dampener and flexplate, water pump, intake manifold and carburetor, and exhaust manifold.

Source:

Hiperformer.com: Long Block Versus Short Engine Block -- Engine Rebuild In-Part or Whole?