eXtended Release and Extended Release. Same thing, different nomenclature. Same as Sustained Release - they all mean the same thing, just depends on how the manufacturer decides to denote it.

Some drugs behave slightly differently based on the notation. In particular I remember Wellbutrin SR and XR were manufactured at different times and behaved slightly differently based on the notation. Wellbutrin was the one med I took where I could tell a difference between the brand and generic.