Google has a tendency to give less weight to dynamic urls than static ones. That means if google sees a .php extension or ?article=123, it will not rank as high as a url which has keywords in it and is a static permanent link. WordPress, by default uses dynamic links and therefore is not SEO friendly. Fortunately, it’s easy to change the links. In the newer versions of WordPress, all you need to do is go to the settings button in the wordpress admin and click on permalinks.
Before doing this, you should think about whether or not you have old links which you want to redirect. If there are old links cached by Google, you will need to use a redirect plugin in order to prevent duplicate content issues. Scott Yang has developed a WordPress plug which is available on his site to do the redirects based on regular expressions.
If you don’t have these kind of links, then all you need to do is log in to the wordpress admin and click on settings. Go to “Custom Structure” and enter /%category%/%postname%/ and click save. This will change all of your links to your posts to a seo friendly format. Make sure that your categories for posts are keyword relevant.