Understanding YSlow: What Matters; What Doesn’t
YSlow by Yahoo! is a great little plugin for Firefox and Firebug. It looks at a page and uses Yahoo!’s Best Practices for Speeding Up Your Web Site to tell you how to make the page load faster. At least that is the idea behind it. In practice, however, not all of its suggestions are useful or even meaningful.
A nice short explanation of Yahoo’s YSlow plugin for Firebug