You are made up of cells. So are dogs and cats and trees and fish. Cells are the basic building blocks of life. The word cell was first coined by the biologist Robert Hooke when he was looking at a piece of cork under a microscope. The cork was divided into sections that looked like monastic cells, so Hooke called the sections he saw "cells".
All cells have many functions, but each has a function in common, which is to help keep an organism alive. There are many different kinds of cells.
Each organism has its own kind of cells. Human cells are different from cow cells or tree cells or frog cells or turnip cells. And even within a specific organism, there are many different kinds of cells. Every human, for example, is made up of about 100 trillion cells -- brain cells and bone cells and stomach cells and many other kinds of cells.
For just one example of the specialization of cells, we can study skin cells -- they are flat and waterproof and are constantly being created in the lower epidermis (the lower layer of the outer layer of skin). Then they migrate to the upper epidermis (the outer layer of skin) where they flake off and die. Here are some other examples: Skeletal muscle cells function to provide movement for animals. Liver cells have many functions – one important one is to detoxify harmful chemicals in the body. Red blood cells function to transport oxygen from the lungs to the rest of the body. White blood cells help to protect us against infection.
There are several ways to classify cells, but the main division is between eukaryotic and prokaryotic cells.