What is SWIFT, the financial system from which Russia could be cut off?

SWIFT, or Society for Worldwide Interbank Financial Telecommunication, allows the smooth and rapid transfer of money across borders. The Belgium-based system links 11,000 banks and institutions in over 200 countries and Russia's the second-largest country in terms of number of users. If SWIFT excluded Russia, it'd become hard for Russian companies to pay for imports and receive cash for exports.