A design pattern is a solution model to a design problem. Here a list of sites I often look at:

http://www.welie.com/patterns is a review of most web design patterns with one by one explanation and screensots.

http://developer.yahoo.com/ypatterns is the Yahoo design pattern library. Very good explanations and concrete exemples and best practices.

http://interface.fh-potsdam.de/infodesignpatterns is an excellent interaction design pattern repository focused as well on application than on web design.

patterntap.com is a great screenshot library of Web design patterns classed by type (ex.: bordrs, breadcrumbs, footers, login forms, comments).

http://www.flickr.com/photos/factoryjoe/collections/72157600001823120/ is very good web design patterns gallery created by a Flickr user.