Kachori is supposed to have originated in Uttar pradesh & rajasthan, In these states it is usually a round flattened ball made of fine flour filled with a stuffing of baked mixture of yellow moong dal or Urad Dal (crushed and washed horse beans), besan (crushed and washed gram flour), black pepper, red chilli powder,salt and other spices.
Delhi has another kind of kachori, called 'Khasta kachori' or 'Raj Kachori'.
A variant includes sweet upwas (fast) kachori, made with potato, coconut, and sugar. Kachoris are often served with a chutney, often made from tamarind, mint, or coriander. Another variant is fried and stuffed with pulses (urad and moong especially) and it is generally found in Kutch region in Gujarat. A kachori stuffed with peas is a delicacy in Bengal.
Some of the variants popular in North India includes a version similar to the Rajasthan version accompanied with a curry made of potatoes and varied spices or even chana (chole), similar to one served in chole bhature.