Again, quoting from the WordPress site:
"Plugins can extend WordPress to do almost anything you can imagine. In the directory you can find, download,
rate, and comment on all the best plugins the WordPress community has to offer."
Wordpress Plugins Site
Just like with the Wordpress Themes, Download them, unzip them, then use Filezilla to transfer them to the plugins folder
located inside the /wp-content folder.
Then in the Dashboard, go to the Plugins Section and Activate them. You can Deactivate them at any time.
Here are some I like.