To understand how trees represent evolution, imagine the tree shown here beginning as a single lineage and growing outwards in a branching pattern over time (click the button to view the animation). That first lineage represents the oldest lineage on the tree — the lineage that is ancestral to all the other organisms on the tree. The tips of the branches (called taxa) represent the youngest lineages on the tree. These taxa are often species — but not always. Some trees represent the relationships among major groups of organisms, species, populations, individuals, or even genes.
The branches of the tree correspond to ancestor/descendent relationships within a lineage: the part of the branch nearer the base of the tree represents the lineage that is ancestral to the part of the branch nearer the tips. The branching points, or nodes, of the tree represent the splitting of an ancestral lineage into multiple descendent lineages through the process of speciation. If you take any two taxa and trace their branches backwards, the node at which the two paths meet represents the point in evolutionary time when their common ancestor split into two lineages.