The data is changing second by second. Apple and Microsoft have switched back and forth a number of times over the past hour, but with Apple's stock up and Microsoft's down, it looks like Apple will likely end the day in the second position.
Market capitalization is calculated by multiplying the share price times the number of outstanding shares. It is often used as a public metric of a company's overall net worth.
This comes less than three months after Apple passed Walmart to become the company with the third-highest market cap. Since that time, Apple's stock has continued to rise (it is hovering between $245 and $246 a share as of this writing, hitting a high of $251 today); meanwhile, Microsoft's shares have declined.
After decreasing in October of 2008 and then again in January of 2009 (when Apple CEO Steve Jobs took a six-month leave of absence), Apple's stock has been on the rise. In fact, it has increased 95% over the last 52 weeks.
It's interesting to look at the company's trajectory over the last decade and its transformation from also-ran computer maker to digital and computer technology behemoth.