These, as the name suggests, are ions that are made of more than one atom. These act like regular ions and will combine with other ions of opposite charge.
These type of ions are shown below:
Nitrate- NO3 1-
Sulphate- SO4 2-
Sulphite- SO3 2-
Ammonium- NH4 1+
NO3 1- + Na1+ -> NaNO3
2NH4 1+ + SO4 2- -> (NH4)2 SO4
Notice the (NH4)2 means there are 2 ammionium ions in the compound formed.