Unsalted butter is more perishable but has a sweeter and fresher taste. Unsalted butter is often preferred in baking as most recipes call for salt already. Salted butter can help to mask flavours that might be absorbed during storage (i.e. unsalted butter might take on the flavour of that onion in your fridge).
How do you know if your recipe is calling for salted or unsalted butter? Generally speaking, if you’re using a Canadian recipe, the standard is salted butter, unless it is stated otherwise. Another sign, is if the recipe doesn’t call for any added salt (and doesn’t specify the butter), salted is likely what you should use.
PRO TIP: Do you store your butter on the counter to keep it nice and soft for your toast? If yes, you should make sure it is salted butter, as unsalted is more perishable and won’t last as long outside the fridge.