The theory goes that Linux, the open-source operating system, will replace Windows as the preferred software to run servers. Over time, the cost advantage of software created by a community of developers would overwhelm the pricey Microsoft (NASDAQ: MSFT) product.
So much for theory. Windows is actually taking share from Linux in the server market. According to TheStreet.com "Microsoft picked up 2 percentage points, bringing its market share to 67.1% of servers shipped during the second quarter." Windows server revenue hit $5 billion in the second quarter compared to $1.8 billion for Linux.
http://www.bloggingstocks.com/2007/08/28/microsoft-beats-back-linux/