Eclipse is probably best known as a Java IDE, but it is more: it is an IDE framework, a tools framework, an open source project, a community, an eco-system, and a foundation.https://www.eclipse.org/eclipse.org-common/themes/solstice/public/images/logo/eclipse-200x200.png
EMF Diff/Merge is a tool for merging models. More precisely, it is a technical component which helps create tools that need to merge models. It consists in a diff/merge engine that is designed to prevent data loss and enforce model consistency during merge, thanks to consistency rules - predefined and, if needed, user-defined. It also includes GUI components.
It is intended to be a booster for developing merge-based features or systems such as model refactoring, iterative model transformations, bridges between models or modeling tools, collaborative modeling environments, or versioning systems.
Check the Wiki page for more information.