In a phylogenetic tree, each node with descendants represents the most recent common ancestor of the descendants, and edge lengths correspond to time estimates.
A rooted phylogenetic tree is a directed tree with a unique node corresponding to the (usually imputed) most recent common ancestor of all the entities at the leaves of the tree.
Here is an example of a rooted phylogenetic tree, which has been colored according to the three domain system [Woese 1998]:
An unrooted phylogenetic tree is, loosely speaking, a tree derived from a rooted phylogenetic tree by omitting the root. More precisely, it is a forest of rooted phylogenetic trees depicted so that the roots are all linked. Here is a thumbnail of an unrooted tree¹:
This shows a phylogenetic tree for myosin, a superfamily of proteins. The full image of this tree can be viewed by following this link: Myosin Phylogenetic Tree
Links to other pictures are given in the Pictures on the web subsection below.
Pictures on the web