A tree is a perennial woody plant. It is most often defined as a woody plant that has many secondary branches supported clear of the ground on a single main stem or trunk with clear apical dominance. A minimum height specification at maturity is cited by some authors, varying from 3 m to 6 m; some authors set a minimum of 10 cm trunk diameter (30 cm girth). Woody plants that do not meet these definitions by having multiple stems and/or small size, are called shrubs. Compared with most other plants, trees are long-lived, some reaching several thousand years old and growing to up to 115 m (379 ft) high.
Several of the trees (Cotinus coggygria, Feijoa sellowiana, & Vitex agnus-castus) in this guide are considered to be shrubs. However, they can be trained to develop into tree-like structure.(Courtesy of Wikipedia)
See a preview of Trees.