Asylum seekers are people who have left their own countries, claiming persecution and seeking a place of safety. They may be granted refugee status in their host country and be able to stay.
Internally displaced persons have been forced to leave their homes because of persecution, war or other threats but remain in their own cuntry. They are often the victims of civil war.
Migrants are people who move from their home to another place. This may be internal migration or international migration, often people seeking a better life in another country. Unlike refugees, migrants are free to return home because, although poor, their lives are not in danger. They are often called economic migrants.
Returnees are refugees who, when conditions allow, return to their own countries. This is known as voluntary repatriation.