There are special use mobile air compressors that have special requirements. Example of these include the compressors used to compress breathable air used by firemen or scuba divers or those used to generate medical grade air in ambulances.
These types of compressors have very specific requirements and as such use a lot of more complicated technology then is required to operate a basic air compressor for industrial of construction applications. You can also buy Mobile Diesel Air Compressors from Chicago Pneumatic.
One can expect to pay a premium for these specialized compressors, which is why they are primary designed for institutional use. Then there are the small compressors used by private people in their homes or workshops.
On the more powerful end, these might include six gallon air compressors used to drive nail guns and paint sprayers; while on the weaker end, perhaps the smallest and weakest are those compressors designed specifically to operate air brushes.
In all of these instances, portability is one of the key features that make them relevant. These kinds of compressors tend to be much weaker and less tough than the industrial or special use compressors, but then again most household applications are considerably lighter as well.
One can find mobile air compressors for as little as $20 for a little airbrush model to many, many thousands of dollars for some of the larger industrial/construction mobile compressors. The range is very large, so keep this in mind when shopping around.