All life on earth is categorized into a big tree based on their relationship to one another. At the top of this tree are the kingdoms of different life. The kingdoms are split into Eukaryotes and Procaryotes. Eukaryotes have organelles—smaller structures inside the cell which form specialized functions similar to organs in an animal. The defining organelle of the Eukaryotes is the nucleus, which houses the genetic encoding for the Eukaryote. Procaryotes, conversely, lack most of the organelles that make up Eukaryotes. In particular, they lack a nucleus, such that their genetic encoding floats around the cell freely. They also lack a mitochondria, which powers the Eukaryotes and might have been more divisive in the Eukaryotes evolution.
The Procaryotes make up two Kingdoms: Bacteria, and Archaea. Bacteria are more familiar to most people. Any bacterial infection is, as its name suggests, bacteria attacking the body. Archaea are more rare to encounter, but likely make up the earliest form of life on earth. Archaea fossils have been found from a time when the earths conditions would not have sustained modern life, and as such, Archaea are often found in the harshest of environments. Some Archaea live in salt water, with a salinity too high for most bacteria, or in thermal ocean vents, with temperatures higher than the boiling point of water.
Eukaryotes make up all animals, plants, and fungus around us. We are a Eukaryote, and so is the yeast that leavens your bread.