What is the process of formation of curd from milk called?and how it is done?

 Formation of curd in milk is called 'Fermentation'. Curd is formed when milk is pasteurised and fermented by a combination of lactic acid bacteria and a protease. Curds are a dairy product obtained by curdling (coagulating) milk with rennet or an edible acidic substance such as lemon juice or vinegar, and then draining off the liquid portion (called whey). Also natural souring by lactic acid bacteria or yeast leads to curdling (sour milk cheese). 
The increased acidity causes the milk proteins (casein) to tangle into solid masses, or curds.

in short :

it is known as fermentation

it done with the help of the bacteria lactobacillus which respires anarobically to produce lactic acid

when we add a spoon of curd to milk, lactobacillus gets added to it.