Nonimmigrant Status
Nonimmigrant status refers to a visa status that is temporary in nature, as opposed to "immigrant" status, which is permanent. All work visas and dependent visas are considered "nonimmigrant" visas.
Application to Extend Status
This application is filed by someone who has been granted a visa and who seeks to extend the period of authorized stay after they have arrived in the U.S.
Application to Change Status
This application is filed by someone who has been granted a visa in a certain nonimmigrant status and wishes to change their status to a different status after they have arrived in the U.S.