Gerrit is a web-based code-review system built on top of git. Its own documentation states it directly: “Gerrit Code Review is a web-based code review tool built on Git version control.” It lets teams examine changes before they are integrated into the main codebase, working both for open-source projects with restricted approval rights and for environments where every contributor can approve.
Gerrit grew out of Google’s work on the Android project in the late 2000s and is designed to scale to thousands of concurrent developers. Its patch-set-based workflow, as its project site describes, “more closely resembles the proven mailing list workflow used by Git itself and the Linux kernel, where patches are reviewed and iterated on before being applied,” rather than the branch-and-merge style of pull requests.
The Android source documentation describes how this works in practice firsthand: “AOSP relies on a Gerrit, a web-based code review system for projects that use Git.” Contributors push a change, request reviewers, and code owners examine it; the interface shows checkmarks for approved files, X marks for rejected ones, and clock icons for pending reviews. If reviewers ask for changes, contributors amend the commit locally and push a new patch set onto the same Gerrit change.
The defining feature is enforcement: under Gerrit, review is not optional. Changes must be approved before they can land in the source tree, which is why large projects such as Android, Chromium, and Qt have used it to keep quality high across very large contributor pools.