Before you can save a version (or "snapshot") of your project with git commit, you first need to tell Git exactly which changes you want to include in that snapshot. This intermediate step is fundamental to how Git operates and provides fine-grained control over your project's history. This process involves understanding the three main states a file can be in relative to Git: modified, staged, and committed.

These states correspond to the three main sections of a Git project: the Working Directory, the Staging Area (also called the "Index"), and the Git Directory (the repository itself, stored in the .git subfolder).

The Three Areas

Working Directory: This is your project folder; the files you see and edit directly. Any changes you make to files here initially mark them as modified relative to the last snapshot stored in the Git repository. Think of this as your workbench where you are actively changing things.
Staging Area (Index): This is an intermediate area, a sort of "preview" of your next commit. It holds information about what will go into your next commit. When you stage a modified file (typically using the git add command), you are adding its current state to the staging area. This indicates you intend to include these specific changes in the next save point. It's like placing items you intend to ship into a specific packing box, separate from other things on your workbench.
Git Directory (Repository): This is where Git stores the metadata and object database for your project, including all the committed snapshots of your project history. When you commit (using the git commit command), Git takes the files as they are in the staging area, creates a permanent snapshot of them, and stores that snapshot in the Git directory. This is akin to sealing the packing box and handing it over to be officially recorded and shipped.

The Lifecycle of File States

Let's trace how a file moves through these states:

Untracked: When you first create a file in your working directory, Git doesn't know about it yet. It's untracked.
Tracked / Unmodified: You add the file to Git's tracking (e.g., using git add and then git commit). Now the file is tracked and considered unmodified because your working directory version matches the version in the Git repository's last snapshot.
Modified: You edit the file in your working directory. Git sees that it differs from the last committed version, so the file is now modified.
Staged: You use the git add command on the modified file. This takes the current version of the file from your working directory and adds it to the staging area. The file is now considered staged. Note that the file in your working directory hasn't changed; you've just marked this specific version for the next commit. It's possible to modify the file again after staging it; in this case, the file would be both staged (the version you added) and modified (the newer changes in the working directory).
Committed: You use the git commit command. Git takes the snapshot of the files currently in the staging area, saves it permanently to the Git directory history, and the files in the staging area are now considered committed. They usually revert to the unmodified state, assuming the working directory version matches this newly committed version.

This workflow might seem like an extra step compared to simpler systems, but the staging area is a powerful feature. It allows you to carefully craft your commits, grouping related changes together into logical units, even if you made other unrelated changes in your working directory simultaneously. You decide exactly what goes into each snapshot.

The relationship between the Working Directory, Staging Area, and the Git Repository, showing the commands that move changes between them.

Understanding this cycle of modifying, staging, and committing is fundamental to using Git effectively. You'll use commands like git status frequently to see which state your files are in.