WordPress.com vs. WordPress.org « Support — WordPress.com:
"The distinction between WordPress.com and WordPress.org can cause some confusion for people. Let’s clear it up. WordPress.com is brought to you by some of the same folks who work on WordPress, the Open Source blogging software. WordPress.com utilizes the same WordPress software which you can download at WordPress.org. With WordPress.com the hosting and managing of the software is taken care of by the team here at Automattic. With WordPress.org you need to install the software on your own server or with a 3rd party provider."