It is important for someone traversing through the git log to easily understand what each commit was all about. Good commit messages usually include a number of a task or an issue in a tracker and a concise description of what has been done and why, and sometimes also how it has been done.

Better messages may look like:

TASK-123: Implement login through OAuth
TASK-124: Add auto minification of JS/CSS files
TASK-125: Fix minifier error when name > 200 chars

Whereas the following messages would not be quite as useful:

fix                         // What has been fixed?
just a bit of a change      // What has changed?
TASK-371                    // No description at all, reader will need to look at the tracker themselves for an explanation
Implemented IFoo in IBar    // Why it was needed?

A way to test if a commit message is written in the correct mood is to replace the blank with the message and see if it makes sense:

If I add this commit, I will ___ to my repository.

The seven rules of a great git commit message

1. Separate the subject line from body with a blank line
2. Limit the subject line to 50 characters
3. Capitalize the subject line
4. Do not end the subject line with a period
5. Use the imperative mood in the subject line
6. Manually wrap each line of the body at 72 characters
7. Use the body to explain what and why instead of how

7 rules from Chris Beam’s blog.