The stock market is extremely safe in the long term (20+ years), not so much in the short term. Don't put next month’s house payments in the stock market; put your retirement money that you won't touch for another 30 years in it.

The biggest advantage with index funds like the S&P, Dow, etc. is that they give you instant diversification and are professionally managed. The largest disadvantage is that they managed for a fee (they keep a certain percentage of your return).

Hope this helps.