Ammonia is a toxic gas common in Earth's atmosphere as well as the atmospheres of other planets, including Jupiter, but you would never use it for cleaning. Instead, you would use ammonium hydroxide, which is ammonia mixed with water. This compound is alkaline, which makes it caustic, and its cleaning action is that of a strong detergent. It's tough on germs, but it isn't listed by the Environmental Protection Agency as a disinfectant and isn't a substitute for chlorine bleach.
How Ammonia Works
Because it's alkaline, like lye, ammonia has the ability to combine with fats and oils and turn them into soap. When you clean with ammonia -- or more properly, ammonium hydroxide -- you effectively create a layer of soap on the surface that, because it's wet, easily wipes away with a cloth. Any ammonium hydroxide left on the surface evaporates quickly without leaving streaks, which is why ammonia-based cleaners are recommended for smooth, polished surfaces, such as glass.
Ammonia Isn't a Disinfectant
The question of whether ammonia-based cleaners kill germs is a complex one. Some household disinfecting cleaners contain ammonia, and cleaners sometimes recommend it as an alternative to bleach. The fact that the EPA does not recognize it as a disinfectant, however, means that you shouldn't rely on it for this purpose. It may kill salmonella and e. Coli, for example, while being ineffective against the staphylococcus bacteria. The same can be said for vinegar, which is also often recommended as a disinfectant.
Never mix ammonia with bleach. The combination produces dangerous toxic fumes.
Even So ...
Reputable sources, such as Utah State University Extension, recommend mixing ammonia with detergent or with vinegar and baking soda to make an all-purpose household cleaner. Such cleaners may be effective disinfectants simply by virtue of their ability to physically remove microbes from nonporous surfaces.
The EPA and other government agencies no longer recommend bleach for mold cleanup. Instead, they advise physically scrubbing the mold with detergent. Adding ammonia to a detergent solution increases the cleaning potential of the detergent and makes scrubbing more effective. It won't prevent the mold from growing back, though.
The Bottom Line
Don't rely on ammonia as a disinfectant. It's a strong cleaner that can make cleaning easier, and it may kill some germs, but if you need sterile conditions, use a registered disinfectant, such as bleach. If you have a product containing ammonia that claims to be a disinfectant, it probably contains other disinfecting ingredients, such as hydrogen peroxide.