Plugins are a way to add additional funtionality to the website. There are many plugins available at http://wordpress.org/plugins/. Most of the plugins on the wordpress website are free. Be aware that plugins are written and uploaded by people on the internet and not neccessarly a reliable company. A plugin may contain a virus, and may be the reason a website crashes or loads incorrectly. If the website is having issues after you installed a plugin uninstall the plugin.
In order to download a plugin, select the plugin you want on the above website. This will bring you to a page similiar to what is below. Here you can preview the plugin and see reviews. Click on the "Download Version XX", this will download a .rar zipped file. You don't have to unzip the file just save it in a place you can find it for later.
Once you've downloaded the desired plugin, the next step is to install the plugin. Hover over "Plugins" and click "Add New".
This will bring you to the screen below where you can Upload the file for the plugin that you downloaded. Click on the "Choose File" , select the file that we downloaded earlier. Click on the "Install Now" button to install the plugin.
To view the plugins that you have installed hover over "Plugins" and click on "Installed Plugins".
Here you can Activate, Deactivate and Delete plugins. You cannot delete a plugin when it is activated. To delete a plugin, first click on "Deactivate", then click on "Inactive" toward the top to view all the inactive plugins. Here you can click on delete to permintely delete the plugin and any files that are associated with it.
Click on "Yes, Delete these files and data", this will delete the files off our server that are related to the deleted plugin.