yogurt is made by adding a live bacteria culture to milk. It is then left for ten to twenty hours and becomes nice and creamy. ou can then save some of this mix back and add that to your next batch.
curd is the solids seperated from milk by using something slightly acid to curddle it, for instance lemon juice. The liquid that is left, is the whey which is not much use, but it can be used in cooking, or fed to animals. Paneer, or cottage cheese is made in this fashion.