According to my friend from Bangladesh, yogurt is what happens when you leave a glass of milk on the kitchen counter for five minutes in India.
He also told me (in a more serious vein) that most families have their own personal culture of yogurt-making bacteria that has been passed down for several generations.
Homemade yogurt = milk, possibly nonfat dry milk added for more body, and yogurt culture (i.e. a spoonful of yogurt from the previous batch). (The NFDM shows up in the ingredients list as “milk solids” or something like that.) Most commercial brands have pectin and stuff in them, to make them more smooth and cohesive, but it doesn’t make a big difference as far as taste goes. And it may be that your favorite Indian restraurant just gets their yogurt in big tubs from their distributor, anway.
If you haven’t tried plain, whole milk yogurt, DO! It’s so amazingly rich and delicious, and especially refreshing on a hot day. If you don’t like it plain, you can add just a little honey, perhaps, or a spoonful of jam . . . Er, but, I suppose that’s another thread.