An employment based green card is a work visa that can be requested in order to legally enter the USA usually based on their level of education or a skill that is needed.
There are several categories like:
E2 and E1 visa for investors and traders
EB-1 Green for Researcher or Professor
EB-2 Green for Skill or Ability
EB-3 Green for Professionals
EB-4 Green for religious workers
H-1B visa for specialty occupations
H-2B visa for non-agricultural workers