Microfinance is based on a very simple principle. The poorest can only improve if they invest, and very small loans may be sufficient to get them started. But conventional banks do not bother with such loans, and informal money-lenders charge horrendous rates. Microfinance step in and lend small amounts, often without collateral in a community-based scheme where one's reputation is sufficient to obtain somewhat reasonable repayment rates. I am not totally convinced this scheme would work without subsidies, but it obviously serves a useful purpose, as long as it does not crowd out the regular financial system.
Beatriz Armendáriz and Ariane Szafarz point out that the latter can become a problem because of mission drift: as microfinance institutions grow, they gradually target larger loans, neglecting their original mission and becoming more like regular banks. This is like car models that grow in size through the years to follow the life-cycle of their drivers. But Armendáriz and Szafarz think that what looks like mission drift could very well be cross-subsidization, and larger and more profitable loans are made to help continue giving small and less profitable ones. The distinction is important, as donors could be put off by mission drift.