What are blood groups?

I have been hearing about the term; ‘Blood Groups’ for long. I will appreciate it if you can kindly educate me about them.

Deinde (by SMS)

A blood group is a category of blood labeling recognized by the two distinct proteins that may or may not be on the surface of your blood cells: we’ll call these Protein A and Protein B. There are four blood groups, A, B, AB and O depending on if these molecules are present in your blood. While each Blood Groups A, B and AB contain molecules, A, B and AB, Blood Group O has no molecule. It is for this reason that the Blood Group O is known as Universal donor because, people with the Blood Group O can give blood to anyone without any adverse reaction.