Typically when baking, you’d use unsalted butter. Using unsalted butter means that you can control the volume of salt you add to your bakes by using salt as a separate ingredient. When a recipe calls for unsalted butter, it usually means that salt is not a required ingredient.
If you use salted butter in your bake even though the recipe calls for unsalted butter, the chances are your bake will carry the salty flavour.
Unsalted butter lasts about 1 month in the fridge, whilst salted butter lasts around about 3 months. This is because salt is a preservative. If you think your butter might be off, give it a good sniff. The nose always knows. Slice your butter. Is the inside the same colour as the outside? If the butter is off, the outside forms a darker casing around the butter.