The link below gives a good description of the difference between SharePoint CU, PU and COD and recommendations for when they should be installed.
Tag: CU
Getting the CU for your SharePoint installation
The pages below shows you how to get information about the CU (Cumulative Update) for your SharePoint installation.
The first page shows you how to get information about the CU.
http://www.brightworksupport.com/find-your-sharepoint-cumulative-update-or-patch-level/
The second page shows you what CU you are using based on the information found through the first page.
http://blogs.technet.com/b/steve_chen/archive/2013/03/26/3561010.aspx