Jie Li, one of the Program Managers for the SharePoint product family recently created a very interesting article revealing the internals about what's going on under the hood when installing updates for SharePoint.
There’re always lots of questions around SharePoint patching. How to patch SharePoint? Which patch should I use to get a certain build number? In which order should I apply those patches? What is CU? What is SP? What is the difference between them?… All these questions are hard to answer – because to understand the answers, you have to understand the patching process of SharePoint.
His article provides insights about these internals: