Installer Plugins are used for installing Extensions on your Joomla powered website.
Extensions can be: Plugins, Modules, Components - or Packages which contain a combination of one or more Component plus Plugins and / or Modules.
There are four methods (or options) for installing the Extension / Package.
Let's touch on each of those - though the method title gives a good idea.
Install from URL
This method allows you to install Packages / Extensions from a URL.
Install from Web
This is, perhaps, similar to Install from URL - this method will load the official Joomla Extensions Directory and allow you to choose from the many Extensions uploaded there. Once you pick the desired Extension you can use the Install button to add the Extension to your website.
Install from Folder
This method allows you to install Packages / Extensions from a folder within your site folder structure - you first need to upload the Package to that folder via FTP. This method is useful if the Package is particularly large.
Install via Package File Upload
This allows you to install Packages / Extensions from your local computer / laptop - this is the method used 99.9% of the time by UsingJoomla.com
Extensions - Install
Extensions - Update
Extensions - Update Sites
Plugin Manager - Edit Installer Group
Plugins Related Help Resources (Generic)
Plugin - Options