What is wordpress ?
WordPress is blogging application
A content management System
A web publishing tool
powers 18% website
WordPress -The application itself
Free to use on any standard web server.
To use wordpress as self hosted application you need domain name and hosting space on a rent.
WordPress.org -place where you can download wordpress package,themes,user forums ,documentation about how to use wordpress.
-WordPress.org site is A self hosted website.
Wrdpress.com- A cloud Hosted service that runs wordPress.
WordPress.com site is site hosted on WordPress.com.