In some cases, when a person immigrates to the United States on the grounds of employment, they do so on a temporary basis. This could be changed over time -- but they are essentially only allowed to be in the U.S. for a finite amount of time. There are other cases, though, where the person has the ability to permanently stay in the U.S. based on his or her employment visa.
For those that fit into the latter group, there are five categories that you need to know about. They run from EB1 to EB5, in numerical order. EB1 preference is for those who are highly skilled or have extraordinary ability. EB2 is similar, reserved for those with "exceptional abilities" or advanced degrees.
EB3 is more general, applying to those who are professionals, skilled workers, or other workers. There are some other qualifications that must be met to show why someone is a "professional." EB4 is a more specialized category for those who work in various professions. Again, some more intricate rules may be in play here.
EB5 is for those who want to make a substantial contribution to a U.S. business as an investor.
Many of these preferences require an I-140 form to be completed by the employer, and there are obviously plenty of major hurdles that must be cleared to allow the visa to move ahead logistically. Anyone who is seeking to permanently remain in the U.S. via a work visa needs to consult with an attorney to ensure that their case is being handled properly.