A Critical Patch Update is a collection of patches for multiple security vulnerabilities. Critical Patch Update patches are usually cumulative but each advisory describes only the security fixes added since the previous Critical Patch Update advisory. Thus, prior Critical Patch Update advisories should be reviewed for information regarding earlier published security fixes.its released on January, April, July &October.
Steps to Apply CPU:
Shutdown all the instance , and oracle services such as listener.
ensure $PATH variable.
Download Patchand unzip it.
Use the Opatch like this : opatch napply -skip_subset -skip_duplicate.
sqlplus / as sysdba
SQL> @catbundle.sql cpu apply
Note : For rac database nothing diffrenece unless you have to shutdown services node by node and patching ( you can patch one node then the other one).