Every user on your WordPress website or blog has a role. WordPress has the following built-in roles: Super Admin (used only in a multisite installation), Administrator, Editor, Author, Contributor and Subscriber. A WordPress user with an administrator role can do anything possible on WordPress, such as creating new articles or pages, installing and configuring plugins to add new functionality to the WordPress website, installing or changing a WordPress theme etc. A user with subscriber role can only manage his or her profile.
Some plugins and themes create additional custom WordPress user roles that are used to give permissions to users to complete specific tasks related to the plugin itself. For example if you install a shopping cart plugin, new roles such as Shop Keeper and Data Entry are typically created. Read the article Use WordPress User Roles for Improved Security for more detailed information on the security best practises of using WordPress users roles.