In its most basic form, a financial instrument is any asset that can be bought, sold, or traded. Financial instruments can be currency, shares in a company or bonds. Financial instruments can also be derivatives, which are contracts whose values are based on underlying assets, and are determined by the rise and fall of that underlying asset.
Financial instruments are the basis of stock markets, foreign currency exchanges, and CFD trading. Traders either trade actual possession of financial instruments, such as purchasing one currency for an equivalent (at the time) amount of a different currency, or shares of publicly traded companies. CFD traders do not take actual possession of the currencies or stocks, but rather trade against the projected price action.