As you look at the trees and shrubs around you, you may begin to wonder about the differences between the two. When is a plant a tree or shrub? At what size is it a shrub? A tree?
First, trees and shrubs are plants that have woody growth. Many people, for example, think that banana trees are trees, but in fact they are not; they are considered to be the world's largest herb. Only plants with woody parts are trees and shrubs.
Once you know that it has woody growth, you can determine whether it is a tree or a shrub. The generally acknowledged definition of a tree, according to USU, is a "woody plant having one erect perennial stem (trunk) at least three inches in diameter at a point 4-1/2 feet above the ground, a definitely formed crown of foliage, and a mature height of at least 13 feet."
Shrubs, therefore, are the opposite : a "woody plant with several perennial stems that may be erect or may lay close to the ground. It will usually have a height less than 13 feet and stems no more than about three inches in diameter."
These definitions serve as good starting points for distinguishing between trees and shrubs. However, as with everything in life, there are exceptions. Some trees, such as river birch and Japanese maple may have multiple trunks. Some shrubs can be shaped into a small tree by training one trunk. However, as long as you follow the general definitions, you should be able to decide whether the plant is a tree or a shrub.
Source: What is a Tree?