Themes define how the public appearance of your web site will actually look, and creates a base of presentational code for the web site to run on. Specialized themes often introduce unique features and functionalities that can tailor the theme to be specialized for a specific purpose such as blogging or small business content management. [...]
Wordpress is completely customizable via plugins, and there are no design restrictions. What you can and can’t do with your site is governed completely by the hosting environment where your web site is located. Additionally, you can also have your own domain name, and integrate Wordpress with other web services and applications to extend its’ [...]
Its features are limited to exactly what you see, and your options for design changes are very limited (and usually not free) as well. Plugins are not an option with Wordpress.com, and you do not have your own domain name. Access to source code is minimal. Affiliate marketing is also not permitted on Wordpress.com.