Apple and Microsoft have always disagreed in how to display fonts on computer displays. Today, both companies are using sub-pixel rendering to coax sharper-looking fonts out of typical low resolution screens. Where they differ is in philosophy.
Apple generally believes that the goal of the algorithm should be to preserve the design of the typeface as much as possible, even at the cost of a little bit of blurriness.
Microsoft generally believes that the shape of each letter should be hammered into pixel boundaries to prevent blur and improve readability, even at the cost of not being true to the typeface.
read full article, by Joel Spolsky, a software developer in New York City
(thank you larry)