Arbitrage
One of the main strategies of trading, traditionally associated with banks, is arbitrage. In the most basic sense, arbitrage is defined as taking advantage of a price discrepancy through the purchase/sale of certain combinations of securities to lock in a profit.
Many people confuse arbitrage with what is essentially a normal investment. The difference between arbitrage and a typical investment is the amount of reward: the risk in what is known as arbitrage today (to distinguish it from theoretical arbitrage, which effectively does not exist) is market neutral. From the second the trade is executed, a profit is locked in. Investment banks, which are often active in many markets around the world, constantly watch for arbitrage opportunities.
One of the more notable areas of arbitrage, called risk arbitrage, evolved in the 1980s. When a company plans to buy another company, often the price of a share in the capital of the buyer falls (because the buyer will have to pay money to buy the other company) and the price of a share in the capital of the purchased company rises (because the buyer usually buys those shares at a price higher than the current price). When an investment bank believes a buyout is imminent, it often sells short the shares of the buyer (betting that the price will go down) and buys the shares of the company being acquired (betting the price will go up).
Read more about this topic: Proprietary Trading