Diversification is considered to be the only free lunch in investing. (If you are new to this concept, check out Introduction To Diversification, The Importance Of Diversification and A Guide To Portfolio Construction.) In a nutshell, by investing in a range of assets, you reduce the risk of one investment's performance severely hurting the return of your overall investment. You could think of it as financial jargon for "don't put all of your eggs in one basket".
According to one interpretation of the efficient-market hypothesis (EMH), only changes in fundamental factors, such as the outlook for margins, profits or dividends, ought to affect share prices beyond the short term, where random 'noise' in the system may prevail. The 'hard' efficient-market hypothesis does not explain the cause of events such as the crash in 1987, when the Dow Jones Industrial Average plummeted 22.6 percent—the largest-ever one-day fall in the United States.