The form of a tree is a phenotype that certain plants take on. Trees are not a single family of botanical lifeforms, but are rather an example of convergent evoloution, where many widely different types of plants have evolved the same basic shape for different reasons. Because of this, "tree" is basically a useless term as a biological classifier, since a fifteen meter tall Black Cherry tree is much more biologically related to a tiny strawberry plant then it is to a phenotypically similiar tree, such as an Alder. Trees can be both gymnosperms and angiosperms, and amongst the angiosperms, trees are almost always dicots, plants characterized by net-like venation, two seed leafs at germation and a generally more complicated structure.

The definition of tree given Webster 1913 is fairly accurate :

Any perennial woody plant of considerable size (usually over twenty feet high) and growing with a single trunk
although there are certain exceptions to these rules. For example, palm trees are not actually trees, since they do not produce true wood. A birch tree is definitely a tree, yet they commonly will have two or three main trunks coming from the same root system. The largest problem is with the size qualification. Quite often, whether a given seed grows into a "tree" or a "shrub" is a matter of chance. Two identical seeds may grow into a large, single trunked tree in one location, and into a low, shrubby bush in another location. Thus, even within a species, or even within genetically identical specimens, the term "tree" does not mean much more then a collection of possible traits.