What is a bush and what is a shrub?

Answer 1

A densely-branched, low-lying shrub is what a bush is, and a low-lying, multiply-stemmed woody plant is what a shrub is.

Specifically, all bushes are shrubs even though not all shrubs are bushes. Bushes are lower-lying than shrubs, which are lower-lying than trees. Bushes gets their dense look from either one stem which multiply branches or from many multiple stems. Shrubs always have multiple stems whose branching is more moderate and therefore less dense-looking than bushes.