Back in the day, there were great features available to WordPress.com users that were not available on self-hosted WordPress installs. Jetpack is a plugin that connects to WordPress.com and enables awesome features, powered by their cloud infrastructure.
Jetpack itself is, and always will be free. Some individual features that Jetpack provides will be free, but others in the future may require payment. Installing Jetpack is free and requires no payment information.
Requirements: WordPress 2.7+