Even the simplest form of life is very complex. A tiny bacterium is a vastly complex "machine" capable of performing some remarkable feats of synthesis, growth, response to stimuli, and all the other signs of life we can recognize.
At first glance these cellular machines appear far too complex for easy understanding. Imagine looking into the insides of a jet engine for the first time and trying to understand how it propels an airplane thought the atmosphere.
But the way an jet engine is constructed has to follow certain rules and principles if it is to function properly, and the way a cell is constructed must follow similar kinds of rules if it is to show all the signs of life. No matter how complicated a cell may appear at first glance, there have to be certain basic principles that apply to its construction, and one of the principles most easily seen is termed levels of organization, or a hierarchy of structure.
Organization of matter into higher and higher levels of complexity is found within all organisms. Simple chemicals are combined into larger molecules, these molecules are then used to construct huge macromolecules, and eventually these giant macromolecules are used to construct the constituents of cells, and cells themselves.
Groups of cells work together to form specialized organs and organ systems and when these are integrated into even higher levels they produce independently functioning multicellular organisms.
Each level of organization is part of the next higher one, and at each level, new properties not present in the previous one emerge. One successful branch of biological science has been the study of these levels of organization - their component parts, the way in which they fit together, how they are organized, and what new properties emerge at each level.
A much simplified chart showing some of the more obvious levels of organization and complexity.