Here are some good introductions to microfinance:
Wikipedia on microfinance
Unitus, pharm a microfinance accelerator, search has produced a very good quality, concise 14 minute free online video (requires free RealPlayer) which describes the basics of microfinance.
Unitus also has a good text intro to microfinance.
Muhammad Yunus, founder of Grameen Bank, has also an intro to microcredit.