Cells are the building blocks of the body. Cells are basically the same, but each cell is controlled by a different part of the DNA they all share. Because of this, cells can perform different functions. Some cells secrete useful substances, some cells transfer electric impulses, some cells contract and expand on command and some cells exist only to give shape to parts of the body. Cells communicate with the cells around them via gates on their surface that let selected substances in an out. Cells need to get nutrition from their environments and they need to expel wastes.
Tissues are collections of cells that do the same thing. One kind of tissue forms the surfaces of most of the things in the body. Bone is one kind of tissue and muscle is another. The brain is made up of another kind of tissue as is the skin. Most of the things we find in the body are made up of patches of tissues working together. Sometimes tissues interact with each other to produce a desired effect. For example, the skin is composed of several layers of almost identical tissue (among other things). The lowest level tissue is composed of stem cells that are always reproducing the next higher level. As the levels of tissue move toward the surface they are programmed to kill themselves. By the time they reach the surface they form a protective layer of dead cells.
Organs are the body parts we can see and recognize--they are groups of tissues that work together for a common purpose. Organs include the brain, the heart, the liver, the kidneys and the lungs. The bones and the muscles are also considered organs. The largest organ in the body is the skin.
Organ systems are two or more organs that work together. Organ systems include the respiratory system, which includes the pharynx, the larynx, the bronchial tree and the lungs. The circulatory system includes the heart, the veins, the capillaries and the arteries. The nervous system includes the brain, the sensory nerves, the motor nerves and the spinal chord. The immune system is one of the most loosely connected organ systems. It consists of the lymph system, free cell agents in the blood system and glands that dispatch protective agents when the body is invaded by foreign microbes.
The organism is the entire animal. The functions of the organism is to eat, eliminate wastes, reproduce and evolve. There is one more level above the organism and that is the population. With animals, this is called population and with humans this has many names. In medicine the study of how a human fits into the population is called psychology.