When it comes to the major US stock indexes, the S&P 500 index is the most highly regarded as a barometer of the overall stock market’s performance and an indicator of how large corporations are performing. With that in mind, here’s what all investors should know about the S&P 500 index, how it works, how you can invest in it, and why doing so could be a smart move. Image source: Getty ImagesWhat is the S&P 500 index?The S&P 500 (also known as the Standard & Poor's 500) is a registered t...