kind of Oracle Patches :

Lets discuss about something Oracle Patches .

There's Four kind of Oracle Patches :

One-off patches (bug fix)
CPU (security patches)
Upgrade patches (bug fixes)
PSU (bug fixes are security patches)
There's another kind called Bundle Patched for windows and exadata.
The most two kind of patch that people get little confused about them is CPU and PSU what are they ? when should i use them ?  is there any different between them ?

First Thing you need to know about them since they have different name then sure it's have different.

CPU:  security fixes each quarter rather than the cumulative database.

PSU : same as CPU  patches but include both the security fixes and priority fixes.Note Once a PSU is applied, only PSUs can be applied in future quarters until the database is upgraded to a new base version.
Which mean you can't Apply CPU and PSU and same database.

Some MOS note that could be Useful :

Introduction to Oracle Patch Set Updates (PSU) 854428.1
Quick Reference to Patchset Patch Numbers [ID 753736.1]
Quick Reference to Patch Numbers for Database PSU, SPU(CPU), Bundle Patches and Patchsets [ID 1454618.1]
New Patch Nomenclature for Oracle Products [ID 1430923.1]

PSU  contain fix for bugs that cause Instance crash,Wrong results and Data Corruption on the other hand Dictionary changes , Major Algorithm changes ,and Optimizer plan changes not fixed by PSU.

to check Applied PSU patched you need to run :
opatch lsinventory -bugs_fixed | grep -i 'DATABASE PSU'
 and if you need to check CPU :
 Select * from registry$history;