It's mostly a bugfix
Find the full version here: OPN 2.3.1

Find the patch here: update 2.3.0 to 2.3.1

openphpnuke-2.3.1-full.* - The whole OPN, with modules and all languages included
openphpnuke-2.3.1-core.* - OPN without the modules
openphpnuke-2.3.1-modules-full.* - Only the modules complete
openphpnuke-2.3.1-module-xxx.* - single modules

If you have already installed 2.3.0 you can use the patch files
openphpnuke-2.3.0-to-2.3.1

We recommend to use the *.tgz or *.tar.bz2 files.
These are much smaller than Zip's. Windows can use theses files without trouble:
*.tgz can be handled with Powerarchiver, Winzip or Winrar.
Powerarchiver or Winrar can handle *.tar.bz2 . A pure " tar for Windows" is available as gnuwin32.

The Changelog you find here

The OPN-Team