“MarketWatch” (http://www.marketwatch.com/story/apple-passes-microsoft-for-second-place-in-sp-500-2010-04-22) is reporting that Apple, as of Thursday, overtook Microsoft and became the second-largest company, in terms of market capitalization, on the S&P 500 .

Apple trails Exxon Mobil, according to Standard & Poor’s index services unit. However, “MarketWatch” notes that the S&P 500 is float-adjusted, so it doesn’t use the full market basis for Microsoft. Apple’s float-adjusted market cap reached US$241.5 billion, surpassing Microsoft Corp.’s index market value of $239.5 billion, said Standard & Poor’s. Exxon still remains in the top spot on the S&P 500 with a market cap of more than $300 billion.

What is market capitalization? It’s an estimation of the value of a business obtained by multiplying the number of shares outstanding by the current price of a share.