Mercurial, often abbreviated as "Hg," is a distributed version control system, similar to Git. It was developed to provide developers with the ability to track changes in source code, manage different versions of a project, and facilitate collaboration in software development projects.

Here are some key features and concepts of Mercurial:

1. Distributed Version Control System: Like Git, Mercurial is a distributed version control system. Each developer has a local copy of the entire repository history, making it easier to collaborate in distributed teams.

2. Commits: In Mercurial, changes are grouped into commits, each of which has a unique identifier and a message describing what was changed in that commit.

3. Branches: Developers can create branches to work on different aspects of a project simultaneously without affecting the main development branch. Merging branches is also possible.

4. Pull and Push: Similar to Git, developers can transfer changes between their local repositories and a central or another remote repository, typically done through pulling and pushing changes.

5. Merging: Merging branches in Mercurial allows for integrating changes from one branch into another, which is particularly useful for incorporating new features or bug fixes into the main development branch.

6. Web Interface: Mercurial often provides a web interface that facilitates tracking the project's history and collaboration. Users can view commits, branches, and more through the web interface.

7. Controlled Distribution: Mercurial emphasizes a straightforward and intuitive user interface and is often considered easier to learn and use than some other version control systems.

Mercurial is used in various development projects and organizations, although Git has become much more popular in recent years. The choice between Mercurial and Git often depends on the individual preferences and requirements of the development team. Both systems serve the fundamental purposes of version control and enable efficient collaboration in software development projects.