Since Yahoo! already explains it so well, I will just quote them:
“Everyone talks about Web 2.0. Talk is cheap. How about some real help? Just in time for Valentine’s day we’re giving you a delicious candy box of tools and guidelines for building cutting-edge dynamic web sites.
Code is cool, but howabout some advice, too. We’ve also got the Yahoo! Design Patterns Library, where our best visual and interaction designers have gotten together and created a set of patterns and guidelines for design in a Web 2.0 world. Here you’ll find ideas on the best ways to implement web site elements, along with pointers to examples and the tools within the Yahoo! UI Library that will let you do it. Today’s Pattern Library is just the beginning — expect this part of the site to grow quite a lot over the coming months.”