Major source of financing and saving without the involvment of any financial institution . And most popular too.
Usually it is a square matrix. 10 people 10 months. 20 people 20 months duration. No interest is involed and you get money on mutually decided date .Pay the monthly installments for the specified period of time.
It is widely used in business. 90% of the time nothing is written and every thing is unofficial. But in business communities they issue reciept for installment.