The Diff module is primarily a tool to compare content revisions making it useful tool when administrating content publishing workflows. It also provides a general Diff Engine used by other modules to compare a variety of things.

As of Drupal 8.x, the core Diff Engine has been added to core to provide configuration comparisons.

Modules with Diff integration

Module 7.x 8.x 9.x Notes
Diff Different Yes No Embeds a side-by-side diff on the content page itself rather than at another URL. Also features support for writer-editor scenarios.
Features Yes Yes Uses the DiffEngine to compare different features.
Revision Deletion Yes No Compatible override of the revisions page.
Rules diff (sandbox) Yes No Provides Rules integration.
Entity Share Diff (sub-module of Entity Share) Dev only Yes Yes Provides Diff of Entities between different website instances (ex: Stage vs Prod) using a webservice to connect the instances.