Often times people substitute butter for shortening when they are cooking or baking, simply because they are so similar. Shortening comes in a variety of flavors, including butter and it can either come in solid or liquid form. These days quite a few recipes call for shortening instead of butter, especially for things like cookies and other types of desserts.
If you do not have any butter in the house but you have shortening, you can use it in place of butter with no problems whatsoever. It is completely interchangeable and you shouldn’t notice much of a difference in the taste. Nutritionally butter and shortening are pretty much on the same level with both of them containing quite a bit of saturated fat.