The term "microorganism" is, in scientific terms, a bit outdated and confused. Scientists, particularly those working in the fields of genetics, evolution and paleontology, prefer to group organisms according to their genetic relationships as much as possible. Microorganisms means simply "small living things", which covers a wide range of related and unrelated beings: after all, who decides what qualifies as "small" or not?
A good analogy to this situation would be food; take bread, for example. There are many different kinds of bread, and a baker or epicurean would value the distinctions between them; however in everyday terms, if you just say "bread", we'll have a pretty clear idea of what you're talking about. This is why a term like "microorganisms" is useful to us in a non-scientific context.
Microorganisms typically include all of the prokaryotes (bacteria and archaea) as well as many single-celled eukaryotes, such as the protozoa, as well as some fungi. One of the most significant uses of microorganisms, particularly for the United States, is in the process of fermentation.
Fermentation is, essentially, a specialized form of decomposition, facilitated by various types of yeast, which are considered to be microorganisms. Fermentation generally involves the conversion of sugars to alcohols (both of which are chemical terms, and do not necessarily involve sugar cubes turning into beer). The use of fermentation has fueled the widely-varied production of alcoholic beverages in the United States, as well as the production of ethanol from corn. Ethanol is a fuel additive (as well as a fuel itself) and has significantly impacted the economies and pollution standards of the United States and other countries since its incorporation. There is ongoing research in attempts to use microorganisms to produce other forms of fuel, as well as to clean up the waste products of fuel use.