tl;dr: Don't worry about it. Interchangeable with regular salted butter.
Sometimes called "sweet butter," unsalted butter is exactly what you'd expect. Many people don't realize this even exists, until a recipe specifically calls for it, or a recipe doesn't specify, and they find themselves in the dairy aisle puzzling over what to buy.
I used to be a purist and obey recipes which called for unsalted. This seemed silly after a while, given that pretty much every recipe also calls for salt, and I often end up adding a bit more salt to taste.
Perhaps you are making something with a simple, delicate flavor, like a very light chocolate frosting. You might want total control over how much salt goes into it. In cases like this, it makes sense to use unsalted butter, then add salt by tiny increments.
In the vast majority of baking, though, it simply doesn't matter which you use. The salt content of regular butter varies among manufacturers, but in general, it is about 0.25 teaspoon per 0.25 cup (1.5g per 2 oz). 1:48 ish. Not a lot.
This all refers to bog-standard grocery store butter, and as usual, my focus is on baking. If you're less interested in baking than you are in finding something lovely to spread on toast, the fancier unsalted butters can be really nice when sprinkled with a bit of equally fancy salt. Again, this is a situation when you want total control over the saltiness of the end product.