Marinade = Lemonade made buy a United States Marine!
Sorry couldn't help myself!
A marinade is for imparting flavors into the product. Brining is a balance of sweet and salt in water, which softens cell walls, this allows more of the natural juices to be held in the walls of the individual cells. Many add flavors to their brines, which would impart flavor while brining. Still the purpose of each is not the same.