There are three areas of knowledge you need to know about:

 

Variation
Everybody in your class is human, in other words they all belong to the same species af animal.

However there are features of each pupil that are different and this helps you tell them apart.
Differences include eye colour, height, gender (boy or girl), weight, blood group and so on.

Differences between living things of the same species is called variation.

Continuous variation
Height is an example of continuous variation.
In your school it ranges from the height of the shortest person to that of the tallest person but could be any value in between.

Any characteristic that can have a whole range of values (like height) is an example of continuous variation.

Discontinuous variation
The human blood group is an example of discontinuous variation.
The ABO blood grouping system allows only four possible blood groups (group A, B, AB and O).

A characteristic that only has a limited number of possibilities (like the blood group) is an example of discontinuous variation.