[This information is referenced in the Brave privacy policy. Please do not edit without consulting with @tomlowenthal.]
Since software development is not an exact science, Brave has multiple, expanding layers of testing & feedback between initial code development and releasing that code in Brave proper.
In short: Release is the real deal. Everything else is just a part of our software development and testing process. Non-release channels exposed to the public out of our commitment to open and transparent development practices. But you shouldn't rely on them! They only exist to support and help us build the released version of Brave.
The initial landing place for code changes. Nightly is updated approximately daily, and includes all recent changes within a few days of them landing in the codebase. This channel is intended for people who work on Brave and are happy to experience frequent changes and breakage. If that doesn't apply to you, you should be very cautious about using Nightly. There's a real risk of destructive bugs or privacy and security problems. Understand that we don't make any promises about the quality of Nightly.
Dev is the first channel which collects changes together into a timed release. This is the first channel which we generally anticipate that people outside of Brave may use. Features arriving in Dev may have had as much as several weeks or as little as several days review on Nightly. Once again, the quality of Dev is not guaranteed, and things which appear in Dev may be substantially changed before release.
Beta represents a near-complete technical preview of the next version of the Brave browser. Beta is our last phase of quality and reliability testing before release. This channel is also the last place that we engage in end-to-end integration testing between the browser and the various online services which it relies on. The principle difference between Beta and release is that our privacy and security promises don't apply to Beta since it can contain new and different features from the released version.
This is the version of Brave which you'll get if you visit https://brave.com/download. This is the completed product, which behaves as described in the Brave privacy policy and the terms of service for Brave Rewards and other online features. This is the only version of Brave which is in scope for our security bug bounty program.
More information about the specific Brave versions moving through the release cycle can be found in the Brave Release Schedule.