In less than two years, Internet Explorer has reduced browser share by more than 15%. Firefox and Chrome are the main winners picking up around 10% and 7% respectively.

The usage share of web browsers is the percentage of visitors to a group of websites that use a particular web browser. For example, when it is said that Internet Explorer has 52% usage share, it means that some version of Internet Explorer is used by 52% of visitors that visit a given set of sites. 

Usage share of web browsers (http://en.wikipedia.org/wiki/Usage_share_of_web_browsers)