Animal cells and plant cells. Fungal cells and bacteria cells. Turns out cell are even more diverse than that. A unicellular organism, like a typical bacterium, grows and operates alone. It may hang out with other cells, but they are all on their own. Some organisms however, are multicellular, and in more complex organisms these cells specialize what they do.
Animal cells specialize in many ways. Some cells transport oxygen (red blood cells), some help fight foreign invaders (white blood cells), and some signals from your hands to your spine (nerve cells).
Plant cells specialize in different ways also. The best example is the guard cell which regulates the exchange of oxygen, water vapor, and carbon dioxide.
Another great example, which can be found in both plants and animals is the male and female sex cell.
A group of similar cells that work together is called a tissue. A group of tissues working together form an organ and a group of organs working together form an organ system.
Muscle cell > Heart muscle tissue > Heart > Circulatory System