My answer on Quora:
The International Monetary Fund (IMF) is a multilateral/international institution whose primary purpose is to be a lender of last resort to member governments.
If one of its member countries is having a financial, banking or related crisis, or is about to have one, the IMF is there to lend that country’s government money to help avert/end the crisis and get it back on its feet.
It also provides technical assistance, publishes research and knowledge, and monitors/surveys the health and stability of the global financial system and the economic health of its member countries.
Some more background on what the IMF does
Like any lender, loans are rarely dispersed without an economic program in place such that the borrowing government can pay back the loan with reasonable confidence and not need another one, via a stable and healthy/healthier economy (this is where some of the IMF’s historic controversy comes into play. Many of the conditions it would attach to the loans it provided to developing countries in the 1970s were not very helpful. Since then, economic policy and research has advanced to better understand what might work and what might not in different contexts).
Related to this, the IMF also monitors the health and stability of the global financial system. It publishes surveillance reports on country exchange rates, macro economic reports on member countries, research papers on macro, financial, fiscal and monetary issues, and other technical expertise (see What are the important annual and biannual reports prepared by IMF?).
IMF operations and lending are overseen by its Executive Board, made up of Executive Directors that represent its constituent countries. Voting power is distributed by how much each country contributes to the IMFs resources. In 2010, member countries agreed on a new distribution of “quotas” that gave rising emerging market economies more weight to account for their increasing weight in the world economy. This new allocation didn’t take place until years later (in short, it was held up by Senate Republicans during the Obama administration, but the rest of the world, including the rest of the US government, was on board).
I’ll stop there, because this could easily turn into a full blown paper on the IMF, its founding as a Bretton Woods institution, its role in global economic governance and in the global economy, which is a bit more than what the question asked.. !