A commit is a command by the user that Bazaar-NG should checkpoint the state of the working tree. Checkpointing a working tree results in a revision which may either end up being stored in the RCS directory for a standalone branch or the repository if it has been set up.
Why "checkpoint" instead of "snapshot"?
bzr also supports an "uncommit" command, which allows you to fix mistakes in commit messages and the commit itself. This command removes the commit from the history of your branch, but the revision data is not destroyed, so if you accidentally commit secret data, uncommit will not remove every trace of it.