Differentiate between CustomMasterUrl and MasterUrl?

MasterUrl is the property which is used to set master pages for all forms and non publishing pages CustomMasterUrl is the property used to set the master pages for publishing sites.

MasterUrl: This master page will be used for system pages (ghosted) like list / library forms, site settings, view all contents pages. CustomMasterUrl: This master page will be used for all publishing pages in pages library or site pages library.

- MasterUrl is used to change the layout of all end user pages while CustomMasterUrl is used for changing the layout of admin pages.