Those are not hard-and-fast rules, so there has been some overlap in extension definitions. Some additional extensions include country-specific extensions, like .us for the United States, and .it for Italy.
When registering a domain, you are not limited to a single extension. In fact, many people choose to register a name with multiple extensions, in an effort to ensure brand continuity across a variety of delivery methods. You might purchase the .com extension for your primary website, and then the .mobi extension for the mobile version of your site.