Phylogeneticists use phylogenies to isolate monophyletic groups or groups composed of an ancestor and all of its descendants. These groups are also known as clades. A monophyletic group will include a fundamental ancestor and all of that ancestorís descendants. A split in the tree called a node will represent this ancestor. All of the organisms on that branch of the tree would be its descendants. Note that organisms located on another branch of the tree would not be within that same clade. Only organisms directly connected to the same hypothetical common ancestor are within the same clade or monophyletic group.
Species next to each other from the same node are known as sister groups. These species share similar heritable traits to one another. For instance, angiosperms (flowering plants) and gymnosperms (non-flowering plants) are sister clades. They are both land plants that share many characteristics in common. They differ in their ability to flower. These species belong to the same monophyletic group: land plants.
For instance, the organism at the stem of the tree and all of the other branches connecting to that stem form one monophyletic group. If you trace the stem to the first node in the tree the hypothetical common ancestor and the branches directly connected to that node are another monophyletic group within that first larger clade. Organisms that have different connecting nodes have different hypothetical common ancestors; they are not in the same clade. They diverged from each other along their evolutionary histories.
Often, you may find small clades nested within a larger clade. For instance, all land plants form one monophyletic group but within that group, angiosperms (flowering plants) form another separate monophyletic group.
Scientists continue to discover and isolate groups of species to this day. A truly wonderful resource for phylogenetic trees is the Tree of Life Project. This project features scientists from around the world who have collaborated in order to build up the entire biological tree of life. This project is still in progress. You can find them on the web.