Patch Set Updates (PSUs) are proactive cumulative patches containing recommended bug fixes that are released on a regular and predictable schedule. PSUs are on the same quarterly schedule as the Critical Patch Updates (CPU). The PSUs serve as a new baseline version for reporting issues to Oracle.

PSU patches are intended to be low-risk. This is accomplished by controlling content and thorough testing. Bug fix inclusion criteria include the following:
• Critical technical issues with fixes that may affect a large number of customers and that are already proven in the field
• Critical Patch Update fixes

PSU patches do not include any of the following:
• Changes that require re-certification (for example, Database fixes that cause optimizer plan changes)
• Fixes that require configuration changes

Each PSU has limited new content, typically ranging from 25 to 100 new bug fixes. Content is selected from a review of patches with high customer downloads and critical issues that have been addressed by Development and Support. There is no direct request process for customers.
Oracle associates a version number to the patch. The fifth number of the database version is incremented for each PSU. For example, the initial PSU is version 11.1.0.7.1, the second PSU for Release 11.1.0.7 is 11.1.0.7.2, and so on.

PSU does not change the Database version. The oracle binaries such as opatch, sqlplus still show the base version like 11.1.0.7.
You can search for the Patch Set Update availability documents by searching for the keywords "Patch Set Update Availability Document" (without the quotation marks) in the knowledge search field in My Oracle Support."