The content component (com_content) is what you use to write articles. It is extremely flexible and has the largest number of built in views. Articles can be created and edited from the front end, making content the easiest component to use to create your site content. Help
Components are larger extensions that produce the major content for your site. Each component has one or more "views" that control how content is displayed. In the Joomla administrator there are additional extensions such as Menus, Redirection, and the extension managers.
The contact component provides a way to provide contact forms and information for your site or to create a complex directory that can be used for many different purposes. Help
The users extension lets your site visitors register, login and logout, change their passwords and other information, and recover lost passwords. In the administrator it allows you to create, block and manage users and create user groups and access levels. Help
Please note that some of the user views will not display if you are not logged-in to the site.