What is the difference between a pre-print and a post-print?
The 'pre' and 'post' refer to the peer review process:
A pre-print is the version of an item that has not yet undergone the peer review process.
A post-print is the version of an item that has undergone the peer review process.
It is best if you keep copies of these versions and keep track of the different versions when you submit your work to a publisher. This will avoid problems in depositing the item later.