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Using wordpress.org means that you either have to have a web server or lease one. So while in some ways that is a disadvantage, it means that you can do much more. For example, you can make changes to the code if you are familiar with PHP and MySQL, and you can use plugins and use and create your own custom themes. Wordpress.com does not give you as many options/freedom, but it means that you don’t have to worry about dealing with a server. Depending on your background and interests, this can be a HUGE advantage. For many people I would say start with wordpress.com, and if you want to later, you can always migrate your content to wordpress.org