Lard is produced from the adipose tissue of pigs. It is made by stewing or boiling. If it is well made, it should have an unobtrusive smell or taste. When fresh, it should have a relatively neutral smell or a slight bacon aroma.
The reason cooking with lard is so popular, is that this fat does not contain trans fats.
Lard contains mostly saturated fatty acids, which are good for your health. Lard is a better cooking option than most commercial vegetable oils and even butter! Also, dishes with lard are rich in flavor.
However, lard can go bad. Like almost all cooking fats, there is a point in time when it expires. The high fat content keeps the lard from going rancid for a while, but sooner or later it will go off and smell bad.
In other words, lard smells bad and goes off, because the fat molecules oxidize or go rancid. Exposure to fresh air can shorten the shelf life of lard. So with proper storage, you'll ensure, that doesn't happen too quickly!
When choosing a refrigerator to store your lard, using an airtight container is especially important.
Another reason for lard to smell is that it can absorb odors, which can affect the smell and taste of food cooked with lard.
This is can be sensed a lot with lard cigars or other lard sweets.
