What Is Ubuntu Pro, and Is It Worth It
Ubuntu Pro is a security-hardened version of Ubuntu targeted at enterprise and corporate users who cannot compromise on stability. It is sold as a subscription, but it is free for non-commercial use on up to five computers.
Ubuntu Linux is free, both as a server and on the desktop. It's released twice a year, in April and October. The version numbers are the year and the month. Every two years, the April release is an LTS, or Long Term Support, version.
The interim releases get nine months of support from Canonical for patches and updates, the LTS versions get five years of support.
Corporate consumers of Ubuntu require enterprise-grade support. They need stability and security. They want as little operating system churn as possible, but they do need patches and updates to keep their systems secure. Needless to say, they'll use the LTS releases, and skip the interim releases.