Investment management firms (who typically manage large accounts on behalf of customers such as pension funds and endowments) use the foreign exchange market to facilitate transactions in foreign securities. For example, an investment manager bearing an international equity portfolio needs to purchase and sell several pairs of foreign currencies to pay for foreign securities purchases.
When trading in the forex market, you're buying or selling the currency of a particular country, relative to another currency. But there's no physical exchange of money from one party to another. That's what happens at a foreign exchange kiosk—think of a tourist visiting Times Square in New York City from Japan. He may be converting his physical yen to actual U.S. dollar cash (and may be charged a commission fee to do so) so he can spend his money while he's traveling. But in the world of electronic markets, traders are usually taking a position in a specific currency, with the hope that there will be some upward movement and strength in the currency they're buying (or weakness if they're selling) so they can make a profit. 
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International parity conditions: Relative purchasing power parity, interest rate parity, Domestic Fisher effect, International Fisher effect. To some extent the above theories provide logical explanation for the fluctuations in exchange rates, yet these theories falter as they are based on challengeable assumptions (e.g., free flow of goods, services, and capital) which seldom hold true in the real world.
More specifically, the spot market is where currencies are bought and sold according to the current price. That price, determined by supply and demand, is a reflection of many things, including current interest rates, economic performance, sentiment towards ongoing political situations (both locally and internationally), as well as the perception of the future performance of one currency against another. When a deal is finalized, this is known as a "spot deal." It is a bilateral transaction by which one party delivers an agreed-upon currency amount to the counter party and receives a specified amount of another currency at the agreed-upon exchange rate value. After a position is closed, the settlement is in cash. Although the spot market is commonly known as one that deals with transactions in the present (rather than the future), these trades actually take two days for settlement.