There are two types of protein in milk - Casein and Whey.  Casein makes up the majority of the protein in milk; about 80%. The two most common casein proteins are A1 and A2 beta-casein. Originally all cows only produced the A2 protein.  The A1 is a variant from a mutation that occurred later. A2 beta-casein is also the main protein found in human breastmilk. This is the most natural milk protein for us and is the most easily digestible. Most commercial milk often comes from breeds that produce the A1 protein which is cited as one of the potential causes of inflammation, digestive problems, allergies, and more. Many people who have trouble digesting most commercial milks discover that their dairy sensitivities disappear after switching to A2 milk. 

Raise your glasses folks, this stuff is what health is made of.