WordPress is infinitely extensible. One of the core philosophies of WordPress is to keep the core code as light and fast as possible but to provide a rich framework for the huge community to expand what WordPress can do, limited only by their imagination.
Plugins
Plugins can extend WordPress to do almost anything you can imagine, in the directory you can find, download, rate, and comment on all the best plugins.
Themes
Find just the right look for your blog.