You can compare the state of your repository across branches, tags, commits, and dates. To compare different versions of your repository, append /compare to your repository's path.
We'll demonstrate the power of Compare by looking at the compare page for a fork of the Linguist repo, which is at https://github.com/octocat/linguist/compare/master...octocat:master.
Every repository's Compare view contains two drop down menus: base and compare.
base should be considered the starting point of your comparison, and compare is the endpoint. During a comparison, you can always change your base and compare points by clicking on Edit.
Comparing branches
The most common use of Compare is to compare branches, such as when you're starting a new pull request. You'll always be taken to the branch comparison view when starting a new pull request.
To compare branches, you can select a branch name from the compare drop down menu at the top of the page.
Here's an example of a comparison between two branches.
Comparing tags
Comparing release tags will show you changes to your repository since the last release.
Instead of typing a branch name, type the name of your tag in the compare drop down menu.
Here's an example of a comparison between two tags.
Comparing commits
You can also compare two arbitrary commits in your repository or its forks on GitHub in a two-dot diff comparison.
To quickly compare two commits or Git Object IDs (OIDs) directly with each other in a two-dot diff comparison on GitHub, edit the URL of your repository's "Comparing changes" page.
For example, this URL uses the shortened seven-character SHA codes to compare commits c3a414e and faf7c6f: https://github.com/github/linguist/compare/c3a414e..faf7c6f.
To learn more about other comparison options, see "Three-dot and two-dot diff comparisons."
Comparing across forks
You can compare your base repository and any forked repository. This is the view that's presented when a user performs a Pull Request to a project.
To compare branches on different repositories, preface the branch names with user names. For example, by specifying octocat:master for base and octo-org:master for compare, you can compare the master branch of the repositories respectively owned by octocat and octo-org.
Here's an example of a comparison between two repositories.
Comparisons across commits
As a shortcut, Git uses the ^ notation to mean "one commit prior."
You can use this notation to compare a single commit or branch against its immediate predecessors. For example, 96d29b7^^^^^ indicates five commits prior to 96d29b7, because there are five ^ marks. Typing 96d29b7^^^^^ in the base branch and 96d29b7 in the compare branch compares the five commits made before 96d29b7 with the 96d29b7 commit.
Here's an example of a comparison using the ^ notation.