Dal tadka from a road-side dhaba in Punjab or piping hot Sambar served in a typical Kerala thali over a pile of rice on a banana leaf! Both these dishes are extremely delicious but are poles apart. Funny enough, their base ingredients are almost similar but they taste vastly different... Reason - Tadka!
The proportion of spices in a Tadka, give the entire dish its taste and flavour. So, while a Punjabi Dal has a Hing-Jeera tadka to start with, Sambar has a typical mustard seed flavored tadka.
In India, there are over thousands of tadkas, depending upon the region, food & culture. In fact, tadka is sometimes so personal that different families of the same culture may change recipes and adapt it according to their individual taste.
Even though there is so much diversity, one can still see a trend in the way regional tadkas are made. Lets look at Bengali cuisine - where almost every dish starts or ends with Panchphoron - a combination of radhuni, mustard, nigella seeds, fennel & fenugreek seeds. Or a Gujarathi cuisine which has a classic combination of Jeera, mustard, chilly & curry leaves in almost all dishes. A typical North Indian dish has a typical Hing- jeera tadka while a South Indian dish will have a mustard, chilly & curry leaf tadka!
These tadkas developed differently over time & have been influenced by the region, temperature, living conditions & of course the spices that grow locally. Many traditional families believe that only the lady of the house should make the Tadka as it defines the entire taste of the dish & thus keeps their culture alive.