A good place to start learning about finance is always http://www.investopedia.com/. Further I can recommend reading about modern portfolio theory and the correlation between risk and return: http://en.wikipedia.org/wiki/Modern_portfolio_theory

Surprisingly, Matt Cutts of Google fame has a reasonable overview: https://www.mattcutts.com/blog/make-money-investing-tips/ and accompanying HN thread: https://news.ycombinator.com/item?id=8675952