Yes, JIRA is free for up to 10 users. So, what's the catch?
There is no catch. It is actually free for teams up to 10 people, and it's practically the full feature version.
There are some limitations, but these are pretty insignifant for most teams getting started. If you have a small team, you can start using JIRA for free. If you're new to JIRA, this is a great way to familiarize yourself with it before you implement it across your organization.
It's not just a free trial. It's free forever.