There are over 200 different cell types in the human body. Each type of cells is specialised to carry out a particular function. Even though there are hundreds of cell types in the body, all of them can be grouped into just four main categories, or tissues. This makes them easier to understand. 

Epithelial cells cover over the interior of hollow organs, like blood vessels or digestive organs, or else form the surface of things, like the skin. Without epithelial cells, you would have no skin to protect your body from injury and would have no stomach to digest your food! Epithelial cells can form an impenetrable barrier which will keep other cells out to prevent infections. 

Nerve cells are specialized for communication. They send signals from the brain to muscles and glands that control their functions. They also receive sensory information from the skin, the eyes, and the ears, and send this information to the brain. There are dozens of varieties of nerve cells in the body, each with their own shapes and functions. 

Muscle cells are specialized for contraction. They pull and tug on bones and tendons to produce motion. They also form the thick outer walls of hollow organs, like blood vessels and digestive organs, and contract to regulate the diameter of these hollow organs. 

Connective tissue cells are specialized fibrous tissue cells found in a variety of body parts, including skin, tendons, ligaments, and blood vessels. They provide structural strength to the body, defend against foreign invaders like bacteria and support growth of other cells.