Imagine a material made of groups of atoms, each group having an arrow.
Ferromagnetism: If the material is in a magnetic field, all groups and their arrows point along the magnetic field.
Anti-ferromagnetism: If the material is in a magnetic field, half the groups and their arrows point against the magnetic field such that net magnetization is zero.
Altermagnetism: If the material is in a magnetic field, the groups may orient along different directions, but their arrows still align such that magnetization is zero.