It all started in January 2005 with Critical Patch Updates (CPU). Then Patch Set Updates (PSU) were added as cumulative patches that included priority fixes as well as security fixes. As of the October 2012 Critical Patch Update, Oracle has changed the terminology to better differentiate between patch types. This terminology will be used for the Oracle Database, Enterprise Manager, Fusion Middleware, and WebLogic.
Critical Patch Update (CPU) now refers to the overall release of security fixes each quarter rather than the cumulative database security patch for the quarter. Think of the CPU as the overarching quarterly release and not as a single patch.
Patch Set Updates (PSU) are the same cumulative patches that include both the security fixes and priority fixes. The key with PSUs is they are minor version upgrades (e.g., 126.96.36.199.1 to 188.8.131.52.2). Once a PSU is applied, only PSUs can be applied in future quarters until the database is upgraded to a new base version.
Security Patch Update (SPU) terminology is introduced in the October 2012 Critical Patch Update as the term for the quarterly security patch. SPU patches are the same as previous CPU patches, just a new name. For the database, SPUs can not be applied once PSUs have been applied until the database is upgraded to a new base version.
Bundle Patches are the quarterly patches for Windows and Exadata which include both the quarterly security patches as well as recommended fixes.
References: New Patch Nomenclature for Oracle Products [ID 1430923.1]