Scientists believe that there are over 10 million different kinds of lifeforms, or species, on Earth. Imagine trying to study and understand the lives, patterns, behaviors and evolution of so many different kinds of organisms. In order to make their job easier, scientists classify living things into groups based on how they are the same and how they are different.
As an example, think of your Halloween candy. On Halloween night after returning from trick or treating, what do you and your friends do? Many kids pour their candy out on the floor or bed and begin separating the candy into piles based on how the candy items are the same and how they are different.
The chocolate goes into one pile, the Smartees™ into another, the Sugar Daddies™ into yet another pile, until all the candies are in the appropriate place. Biologists do the same thing with lifeforms. Instead of dumping all the lifeforms on the floor and placing them in piles, scientists write down which group each type of organism belongs to in books.