The Jet Pack plugin is actually a bundle of 16 plugins that have been developed for WordPress.com. Most are free, others are premium plugins that cost money to use. Each plugin can be activated or deactivated according to your needs. If you use a lot of other plugins on your site, you might want to be conservative in what you activate in JetPack – some of your existing plugins may conflict with JetPack’s plugins. (For example, if you have Simplier IPaper activated on your site, and you try to activate Shortcode Embeds, you will get an error. You’ll need to first de-activate Simplier IPaper.)
The screenshot below shows JetPack’s main page:
To use JetPack you’ll need to have a WordPress.com account. This is easy to get, and does not even require starting a WordPress.com blog. Just register here, get your id/password, and you are ready to activate Jet Pack.
JetPack provides a nice stats plugin that you can use to track your readers. It also provides Latex support for mathematical notation, and a bunch of other plugins, many of which are similar to other plugins on the Commons. Take some time and explore.