The Beta cluster is a functional, production-like environment in Wikimedia Labs suitable for final-stage testing of new features. It runs off git master and has its MediaWiki core and extensions code updated about every three minutes.
The aim is to provide an exact copy of the WMF production cluster (including db hosts, apaches, logging, squids, varnish caching, image scalers, etc.) using Wikimedia Labs instances.
See the workboard in Phabricator.
Testing changes on Beta Cluster
The beta cluster uses the same wmf-config directory in the MW core repository as production, but in addition the beta cluster machines load
extension-list-labs files so you can have settings that only apply to Labs.
If you need to run a maintenance script on the Beta Cluster, ssh to deployment-tin.deployment-prep.eqiad.wmflabs (proxying through bastion.wmflabs.org).