Finely cut green chilies and ginger and a phodani (tempering) of mustard seeds, turmeric and salt are added to a mash of boiled potatoes, and after dipping patties of the mash in an herb-seasoned batter of gram flour, the patties are deep-fried. Vada pav is typically served with a chutney (sauce) which is commonly made out of shredded coconut "meat", tamarind pulp, and garlic. Variations of the above basic dish include "cheese vada pav" (where slices of cheese are added); there is "schezwan vada pav" (where the pav is layered with schezwan sauce); "samosa pav" (where a samosa is used instead of a vada); and "Jain vada pav" (where vada ingredients do not include onions, garlic and potatoes). Vada pav served in the nearby state of Gujarat is usually fried in butter or edible oil. The pav is first fried in a mixture of butter or oil and dried red chili powder. After that the chutney is applied in the hot fried pav and the vada is placed in between. This is the only difference between vada pav in Gujarat and Maharashtra. In the state of Gujarat, the original un-fried vada pav recipe is referred to as "Bombay vada pav". This dish was initially started as the most cheapest form of a meal for low income group, but due to its taste, this dish became so popular that many sophisticated hotels also have started dishing out this dish at extra cost. However, this dish is still regarded as a humble dish, and many people have it at breakfast, lunch or even dinner as well.
From the source of http://en.wikipedia.org/wiki/Vada_pav