Basically, the difference is that doing edits in ACR(or Light Room) on RAW FILES, is that you are doing non destructive editing on metadata. Once the file has been rendered into PS you are working on pixels and data is lost once you close the file. If you edit RAW in ACR, you can always go back to make further edits because until you open the file up in PS, you are not doing pixel damage, only edits to a set of instructions.
If you are shooting jpegs,(both ACR & LR can work on jpegs) then you image has already been rendered and any edits you make will be just as if ther were in PS.