WordPress
Jetpack
Jetpack is a plugin from Automattic that connects a self-hosted WordPress site to WordPress.com and adds modules such as stats, backups, search, and site security. Some features run on Automattic's servers, not yours.
How it is measured
Check what is on from Jetpack > Settings and Jetpack > Dashboard in wp-admin. The plugin stores a connection token and the list of active modules in options; wp jetpack module list shows them from the command line. Each active module adds PHP, scripts, or both to your pages.
To see its cost, load a page with Query Monitor and look at requests and scripts attributed to the jetpack plugin. Disconnecting in Jetpack > Settings > Connection removes the link to the WordPress.com account.
Worked example
A woodworking blog on shared hosting activates Jetpack to get stats and image CDN. Page weight on the homepage rises by 140 KB because the Related Posts and Sharing modules load their own CSS and scripts. The owner never used either.
Turning off Related Posts, Sharing, and Likes cuts those two files and a 120 ms render delay. The image CDN stays on and stats continue to appear on WordPress.com.
How it differs
Jetpack is a bundle of features, and WordPress is the CMS it plugs into. The WordPress installation works with or without Jetpack, and its core has no dependency on a WordPress.com account. Jetpack also differs from Akismet, which does one thing, spam filtering, while Jetpack contains many modules that overlap with other plugins you may already run.
Common errors
Installing it for one feature and leaving every module on. Running two stats plugins and doubling script weight. Changing the site URL and breaking the connection, which puts Jetpack into safe mode. Assuming backups are included in the free plan. Forgetting that a connected account has access to the site.
In practice
Open the module list and turn off anything you do not use. If you only wanted one function, such as uptime alerts or backups, compare a single-purpose plugin. After any domain or HTTPS change, reconnect Jetpack and confirm the status in the dashboard.