Scientists like to sort things, and with all the millions of species on Earth, it's an important job. One major way that we classify living things is to break them into 3 domains or branches:

The eukaryote branch includes us, along with other animals, plants,fungi, and one-celled species such as paramecia and amebas. We have big cells and our DNA is found in the nucleus of each cell, surrounded by a membrane.

The prokaryote branch includes bacteria, like the ones that live in and on you. These can protect and help you or they can cause infections, depending on their type. Other bacteria don't live in or on other species; we call them "free-living." Prokaryotes don't have cell nuclei and their organelles don't have membranes. They are one-celled (although these individuals are sometimes attached to each other) and tend to be much smaller than eukaryotic cells.

The Archaea domain of life is probably not one you have heard much about. They were apparently the ancestors of the modern bacteria. They live in places where you might think nothing can live, like the boiling water in hot springs and ocean vents. They can live in very acidic or alkaline water and in other harsh environments. They may be critical in helping cows and termites digest plant material.

Why do you think it has taken such a long time to study these organisms?Can you think why it would be difficult to find and raise them?

You're probably thinking of the Archaea, Bacteria, and Eukarya, the three "domains" of life. Although domains are a useful concept, dividing all of life into three groups like that isn't really correct. The primary reason for this is that one of the domains the Eukarya - is a sort of hybrid between the other two. Eukaryotic cells contain internal structures that were formerly free-living cells that have been incorporated into the eukaryotic whole: the nucleus is descended from the Archaea, and the mitochondria and plastids (chloroplasts and similar structures) are descended from Bacteria. As a result, treating the Eukarya as a separate "branch" on the evolutionary tree doesn't really describe the true situation: eukaryotes are a union between the other two domains, not a third domain independent of the other two.

The answer to this question has changed a lot in recent years. This is because we have discovered lots of new kinds of organisms and because we've gotten much better at determining how similar organisms are to one another. When I was first learning about biology (way before you were born), we learned that the main branches were bacteria, plants, and animals. Since then, scientists have started using DNA to answer this question. You've probably learned a little bit about DNA; it's a molecule in every cell that gives the blueprint for that cell (and the entire organism). It's possible to use computers to compare DNA from different organisms and determine how similar they are to each other. This helps us decide which organisms should be together on the same "branches" of the tree of life. For example, we now know that fungi are more closely related to animals than they are to plants.

Using these tools, we now think the three main branches of life are Archaea, Eubacteria, and Eukaryotes. Archaea are very strange single-celled organisms that often live in unusual places, like volcanic vents on the ocean floor and geysers. Instead of getting energy from the sun (like plants) or from eating other organisms (like animals), they can feed on chemicals that seep out of the earth's crust. Eubacteria are also single-celled, and this is the group that holds most of the bacteria in the world. This includes all of the ones that cause disease (anthrax, meningitis, strep throat) as well as all of the beneficial ones that live on us, in us, in the soil, and all over the place. The Eukaryotes are all of the other living things - plants, animals, fungus, protozoans, algae, etc. Some of these things are single-celled, too (like many algae), but this group also has all of the multicellular organisms. Every organism in this group is more closely related to other organisms in this group than to the other groups. That is, a dog is more closely related to a tree than to a bacterium.

A good website to explore all of the branches on the tree of life is tree_of_life

I hope this answers your question!