A short block is a motor that is assembled with crank, rods, pistons and sometimes the camshaft (not usually). A long block is a motor with crank, rods, pistons, cam, timing chain, heads, valves rockerarms and pushrods (whole valve train).
To make life simple if you buy a short block, you a complete bottom half of the motor (w/o oil pan and accessories) a long block would include the valvetrain and heads.
A tall deck or short deck blocks are blocks that are slightly taller bores. This will give more room for a longer stroke.
a short block is everything below the heads.
A long block includes the heads and sometimes the intake, but usually does not include the intake.
They are just terms used to describe engine assemblies.