What exactly is teeth demineralisation?

Surface Mineral Loss: Demineralization is exactly what it sounds like...A loss of minerals from the surface of the enamel (the outer layer of a tooth). Minerals are lost constantly from acidic foods/drinks as well as bacteria. They must be continually replaced by saliva and fluoride-toothpaste. As long as you add more back, versus what's being lost, you'll be okay. But, it's a constant battle to prevent cavities.

Decalcification: I'm going to assume you mean decalcification. Tooth decalcification is when your tooth looses calcium, looses it's hardness, and becomes more susceptible to decay. Most of the time this condition is due to plaque build up. This plaque has a high bacteria content which releases acids onto the tooth damaging the surface and decalcifying the tooth. In early stages, a flouride application will help.

Mineral breakdown: The tooth is composed a mineral componet (hydroxyapetite) and organic component (collagen). When acid comes in contact with the tooth, the tooth breakdown via loss of the mineral component known as demineralization.

Decay of enamel: Acid, whether from diet (soda, lemonade, citrus fruits), gut (reflux), or plaque bacterial conversion of sugars dissolves the calcium-phosphorus crystals of the tooth structure in much the same way that vinegar (acetic acid) will dissolve chalk (calcium). The reaction can reverse and form mineral crystals if the acid is removed and extra calcium is provided. Fluoride helps build acid resistance.