We usually think of shrubs as bushy and woody plants - larger than the average plant, but smaller than trees.
As with trees, shrubs are both woody plants.
Smaller plants are generally considered herbaceous and more fleshy.
The easiest way to define a shurb is that they have several main stems growing from ground level, (rather than a single trunk). It is this main characterist that enables many of them to become hedges as they merge into each other.