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What exactly is a “derivative”?
The basic idea behind a derivative is very simple actually.  It is a contract where the final value depends on (is derived from) the value of something else.  Stock, for instance, is not a derivative because the contract itself is actually ownership of part of a company.  Whereas car insurance is a derivative because the payout depends on the value of something else namely your (and other peoples') cars. The problem with such a simple definition is that it covers such a broad class.  It covers simple contracts like Futures where the end value just depends on the price of something on a future date.  But it extends to contracts complicated enough that people in finance call them Exotics. Derivatives are broadly used for two things reducing risk (sometimes called insurance) and speculation.  A farmer can use derivatives to make sure she gets paid a certain amount for her corn.  A banker can group a bunch of loans together and sell slices to reduce the pain of a particular loan failing.  At the same time people can use the same instruments to speculate on the price of (for example) that corn or those loans and the main advantage is that they don't have to buy the corn or loans directly.  Any farmer will tell you corn can be very expensive to store. Derivatives generally cause problems both individually and sometimes world wide when people don't properly understand the risks involved.  The most famous example being Mortgage-backed Securities and the recent Great Recession.  You can start understand the instruments and their risks by this wonderful Wikipedia article and later perhaps a used collection of CFA books which cover derivatives in great detail. Edit: Michael Grünewald mentioned Hull's text on derivatives a wonderful middle ground between Wikipedia and the CFA books that I can't believe I didn't think about myself.