Yoghurt (or curds) is widely used in Indian foods. It is an excellent source of milk protein for vegetarians and has relatively low fat content. Natural yoghurt also contains live cultures which are very good for digestion and your stomach generally. Buttermilk is a slightly different and watered down version of yoghurt. Yoghurt is also good for your skin when mixed with a little turmeric and besan flour. It is also, like ghee, one of the five nectars offered to the gods and placed on shrines.
The yoghurt that is bought in the shops tends to be very salty and most of it, in the States at least, is revolting. Making yoghurt takes some time but it is definately worth it. If you cannot be bothered, a relatively good brand is Stonyfield Plain yoghurt- whole milk or low fat but not no-fat. To make yoghurt, you will need a starter. This basically means that you need to already have some yoghurt. Many of the yoghurts in the U.S have essential ingredients taken out so that you cannot make your own. Be sure to buy a yoghurt that has active cultures. I have not yet made yoghurt with a popular commercial brand in the States so I cannot recommend a good one. I suspect that Stonyfield may work. Then again, it may not. It is probably easier to buy yoghurt from an "ethnic" deli. Almost any Greek one will sell good, natural yoghurt. Here is my method for making 1 liter of yoghurt.
Method
Put one litre of milk in a large saucepan and, on medium heat, bring to boiling point. (This is where the milk is just hot enough to start boiling but has not yet). Turn off heat and let milk cool to the point where it is comfortably warm enough to pass over your wrist ( not your fingers which are naturally more insensitive). If the milk is too hot the cultures in the starter will die and if it is too cold, they won't do anything. Into the milk, put two to three tablespoons of yoghurt. If you would like your yoghurt to be slightly thicker than normal, put some milk powder. How much is a matter of experimentation. I usually put in three tablespoons or so. Stir gently and then pour into an earthenware or ceramic pot which has a lid. This last point is very important. If you leave the milk in a metal pot there is a good chance, especially in the American winter, that it will not form yoghurt. This is because metal more naturally conducts the heat from outside and does not provide enough warmth for the culture to survive. Once you have poured your milk into the earthen pot, cover it and wrap up with a blanket. Place in a warm and draught-free place. Leave for twelve hours. If the yoghurt is successful, then place in the refrigerator. If not, throw it out or use it in cooking something. The yoghurt will be at its best after two days.