Ammonia isn't as popular as a cleaning agent as it should be because many people don't like the fumes. It's cheap and efficient when you have a tough cleaning job involving grease or unknown gunk, though, and since manufacturer's have added scents, it's not even quite as smelly.
Those famous fumes are the real gunk-loosening power of ammonia. To use them, put a small amount of ammonia in a container that can be closed - like your oven or a plastic bag. To minimize the use even more (and maximize your savings), put it in a shallow container like a pie pan (glass only) in the bottom rack of your oven, then put drip pans, electric skillets or anything else that's coated with grease or baked on gunk. Close the oven door and leave it all there overnight. The gunk will be loosened by morning so a quick scrub will have everything, including the oven, shining.