Let’s take a quick top-level look at the folders and files that are included within a typical WordPress install.
In the root directory, there are a few core WordPress files. Unless you are digging around in the core WordPress code looking for hooks to use or certain functionality, the only file you may need to ever alter is wp-config.php. You should never alter any other core WordPress files. Hacking core is a bad idea because you won’t be able to upgrade WordPress when a new version becomes available without overriding your changes. The only directory you should need to interact with is wp-content because it contains your plugins, themes, and uploaded files.
This directory contains core directories and files for managing the WordPress admin interface. Another key file in this directory is admin-ajax.php, which all AJAX requests should be run through.
This directory contains core directories and files for various WordPress functionality.
This directory contains subdirectories for the plugins and themes you have installed on your site and any media files you upload to your site. If you create a plugin that needs to store dynamic files of its own, it is a best practice to place them somewhere in the wp-content folder so they are included in a content backup.
The following directories are subdirectories of the wp-content directory.
Any WordPress plugin you install on your WordPress site will be located in this directory. By default, WordPress comes with the Hello Dolly and Akismet plugins.
Any WordPress themes you install on your WordPress site will be located in this directory. By default, WordPress comes with the Twenty Eleven, Twenty Twelve, Twenty Thirteen, and Twenty Fourteen themes.
Once you start uploading any photos or files to your media library, you will start seeing this directory being populated with those uploaded files. All uploaded media is stored in the uploads directory.
In WordPress, you can force the use of any plugin by creating a mu-plugins directory inside of the wp-content directory. This directory does not exist unless you create it. The “mu” stands for “must use,” and any plugin you put in the mu-plugins folder will automatically run without needing to be manually activated on the admin plugins page.
Must use plugins are especially useful on multisite installs of WordPress so you can use plugins that your individual network site admins won’t be able to deactivate.