Phylogenetics is a branch of biology which focuses on the study of the relationships between organisms, both living and extinct. By looking at such relationships and drawing connections between the creatures on Earth, we can learn more about Earth's evolutionary history, and even think about the direction in which evolution might be headed. This branch of biology is at least as old as Linnaeus, who pioneered the taxonomic system used to classify organisms today.
There are a number of ways to think about phylogenetics. At its simplest, phylogenetics involves the construction of family trees, looking at the ancestors of modern organisms and tracing traits through millions of years of evolution. For example, a phylogeneticist might look at a bird and think about ways in which the bird is similar to and different from other organisms. Using this information, the researcher might draw conclusions about which birds are closely related to the bird he or she is looking at, grouping them together and then thinking about the ancestors of that group.
This field of biology looks both at how evolution happens, and why. Phylogenetics involves the inspection of the fossil record, specimens kept in zoos and natural history museums, and living organisms in the wild, from bacteria to elephants. By looking at how organisms are similar and different, scientists can set up a family tree, classifying the organisms and figuring out how long they have been on Earth. In the process of these classifications, scientists also contribute to the field of taxonomy, which allows us to precisely identify organisms, down to their genera and species.
Evolutionary history is quite fascinating, as any glance at the fossil record will reveal. Phylogenetics looks at evolutionary failures and successes, and what might have brought these events about, and it also looks at major shifts in evolution, like the time when organisms first made the shift from water to land, setting the stage for the emerge of humans.
Phylogenetics can be used to look at sweeping history from the emergence of multicellular lifeforms to the modern day, and it can also focus on much shorter periods of evolutionary time. For example, phylogenetics has allowed us to construct the history of the human race, from the earliest hominids to ancestors who were quite close to humans, to modern humans. This history is constantly being augmented with more information, like fossilized hominid remains which are used to fill in the gaps in our understanding.