Here's an excerpt from the announcement:
OJS 2.3.0 introduces a major rewrite of core aspects of PKP applications that reconciles common code (e.g. shared between OJS, OCS, and the Harvester) into a separate library called the PKP Web Application Library (WAL). Many parts of the system have been changed in a way that is transparent to users but that will vastly improve maintainability and the ease with which PKP can deploy fixes and new features across multiple applications. Wherever possible, this has been done in a way that minimizes code breakage e.g. for modified installs of OJS and custom plugins.
Read more about it at "OJS 2.3: What's New."