both functions are used for assigning object reference.
however createObject function creates new object
while the GetObject function is mostly used to assign a reference to existing object though you can also create new obects with it.
Dim xlApp As Excel.Application

'if excel is running then

Set xlApp = GetObject(,"Excel.Application") 'xlApp references existing obj
Set xlApp = GetObject("","Excel.Application") 'xlApp references new object

'if excel is not running then

Set xlApp = GetObject(,"Excel.Application") 'give Error
Set xlApp = GetObject("","Excel.Application") 'start the excel and xlApp references new object