Pre-commit reviews are where code is reviewed before it goes "live" in some sense. Often, this means that reviews happen before the code is committed to a central repository or main-line branch. This means that code does not affect anyone but the author until reviewers have signed off on it, keeping the tree stable and avoiding lots of fix-up changes.
Post-commit reviews happen after code is committed to a central repository. This makes the code available for people to use and develop against while the review process occurs, but it also means that large-scale changes can be difficult to make without disrupting others. Post-commit reviews sometimes happen by individuals or a large group sitting together and reading over the code in a shared setting.