WordPress is a free and open source blogging tool and a dynamic content management system (CMS) working on PHP and MySQL. It have number of features from good templating system to having wordpress plugins to add additional functionality to your wordpress blog.
Most important part of using wordpress is that wordpress community is very active. WordPress developers are creating good wordpress themes and plugins which you can use for your website.
If you are looking for a wordpress theme then you can search and best wordpress theme for your website from wordpress theme repository which contains more than 1500+ free wordpress themes.