"Surówka" is a salad that is made only of fruits or vegetables which are not processed. You don't boil or fry them for example. "Surowy" means "raw".
"Sałatka" is a salad that is made of fruits and vegetables which can be processed and contains some extra ingredients (e.g. meat, pasta, mayo).
"Sałatka" can be considered as a separate dish, but "surówka" is more a side dish.
Hope this helps.