What is the difference between a tree and a shrub?

Trees and shrubs both have wood parts, while other kinds of plants, such as herbs, do not. A tree has a single perennial straight trunk that is, at the height of 4.5 feet above the ground, is at least three inches in diameter. In addition, a tree must reach at least 13 feet at its stage of mature growth (some definitions state that the tree must reach 20 feet and that its trunk has to be two inches in diameter at 4.5 feet above the ground). A shrub, on the other hand, has several perennial trunks that may either grow straight up or may grow near the ground.  A shrub can also have several stems coming from its base. The trunk of a shrub is less than three inches in diameter and its height is lower than 13 feet (or 20 feet, according to some definitions) when it is fully grown. 
https://www.nps.gov/noca/learn/nature/treesandshrubs.htm