When one enters the U.S., he or she is given a certain status based upon the reason for that specific visit. Someone with a student visa, for example, is given the status of a student that allows him or her to study in the U.S.
When a person who has a non-immigrant status wishes to apply for a green card – this is called adjustment of status. For example: someone who entered the US in student status and thereafter, weds a US citizen and applies for a green card based upon that marriage.
Please note that changing from one type of non-immigrant status to another non-immigrant status is called change of status. For example: changing from a student status to H-1B status (temporary worker status).