Rotating Savings and Credit Associations (ROSCAs) are essentially a group of individuals who come together and make regular cyclical contributions to a common fund, which is then given as a lump sum to one member in each cycle. For example, a group of 12 persons may contribute Rs. 100 (US$33) per month for 12 months. The Rs. 1,200 collected each month is given to one member. Thus, a member will 'lend' money to other members through his regular monthly contributions. After having received the lump sum amount when it is his turn (i.e. 'borrow' from the group), he then pays back the amount in regular/further monthly contributions. Deciding who receives the lump sum is done by consensus, by lottery, by bidding or other agreed methods.
See "A Typology of Informal Credit Suppliers"