Apply the modifications in the working copy to the repository as a new changeset.

This is the operation that actually modifies the repository. Several others modify the working copy and add an operation to a list we call the pending changeset, a place where changes wait to be committed. The commit operation takes the pending changeset and uses it to create a new version of the tree in the repository.

All modern version control tools perform this operation atomically. In other words, no matter how many individual modifications are in your pending changeset, the repository will either end up with all of them (if the operation is successful), or none of them (if the operation fails). It is impossible for the repository to end up in a state with only half of the operations done.  The integrity of the repository is assured.

It is typical to provide a log message (or comment) when you commit, explaining the changes you have made. This log message becomes part of the history of the repository.