Network-wide plugins perform an action globally across all sites in the network. Sometimes, you might see these referred to as sitewide plugins, which is old WordPress MU terminology (pre-WordPress version 3.0). Because you're working with one codebase, only one copy of a plugin is needed. All sites within the network use the same copy.
Any changes made to this copy affect every site within your network.
A special breed of plugins — the “Must-Use” plugins — get installed into the /wp-content/mu-plugins folder on your Web server. Any plugin file placed inside this folder runs as if it were part of WordPress. The plugins in this folder automatically execute, without the need for activation in your Dashboard.
You cannot access the files in this folder from the back WordPress Dashboard. If you use the Install Plugins page (PluginsAdd New) to find and install a Must-Use plugin, you may be required to move the plugin files from the plugins folder to the mu-plugins folder. The plugin's readme.txt file will always state if the plugin needs to be moved into the Must-Use (mu-plugins) folder.
Generally, plugins placed in the /wp-content/mu-plugins ...