Dosai is the authentic spelling and pronounciation. Dosa is the anglicized version.
It is prepared by laying a thin circular film from a batter of rice and lentil flour mixed with water on a flat, preheated pan, and frying it with a dash of edible oil or ghee. It gets evenly fried on one side to obtain a brownish tinge. The other side is partially fried next by turning the dosa over. The end product is neatly folded and served.
The crispness, the colour, the amount of batter used and the time for which the batter was allowed to ferment prior to cooking determine the variety of Dosa you eat. A stuffing inside a folded Dosa is not uncommon. A common variety - Masala Dosa is essentially a freshly cooked, crisp Dosa stuffed with mashed potatoes lightly cooked with fried onions and spices. Sometimes the onions are mixed into the batter itself.
Dosas can also be made from cream of wheat ('Maida') in which case they are called 'Maida Dosas' or 'Rava Dosas' (when prepared with 'Rava' or Semolina).
Traditionally, Dosa is served hot with Sambhar - a thick, spicy lentil soup and varieties of chutney - a semi-solid paste usually made of coconut, cereals, parsley, mint and green chillies.