Plugins for WordPress are created to enhance your site and add extra functionality to it. For instance, you might want to have the ability to allow your readers to rate your posts, or to create an image gallery, or get WordPress to do your laundry (yeah rite!).
Most plugins are free – a good place to start downloading them from is http://wordpress.org/extend/plugins/
Step-by-step walkthrough to install a WordPress plugin
1. Download the plugin you wish to install. Unzip it on your machine.
2. Open your FTP client (Filezilla – highly recommended, and free!)
3. Connect to the hosting server (your webhost should have given you the FTP information, or else shoot them in the foot)
4. Once connected, look for the wp-content folder, double click on it. Then look for the plugins folder – that is where you will transfer the downloaded plugin to. Double click on the plugins folder and you should at least see “akismet” which is a default plugin supplied by WordPress.
5. Upload the directory that was created when you unzipped the plugin earlier. You can right click on it, and select Upload.
6. Once uploaded, we need to log into the WordPress administrative Dashboard and click on Plugins in order to activate it.
7. In my case, I’m installing a plugin I bought for USD1 called Ajax Comments. As such, look for the header Inactive Plugins, and hit the Activate link.
8. That’s it, really. But for some more complex plugins, you may have to configure them further. Click on the Settings tab which is on the administrative Dashboard and you’ll see the name of the plugin that you installed. Tweak to size, and Save!