What is the difference between polyatomic ions and compounds? I don't understand why things like hydrogen phosphate are polyatomic ions but things like potassium nitrate are compounds. Can someone explain to me what the difference is, besides that ions are charged?
The ions, by themselves, are charged and thus cannot be complete compounds which are electrically neutral.

We sometimes do have names for polyatomic ions as If they are compounds, like cyanidin (which is a cation containing a pyrylium ion function).  But we understand that a "generic" counterion is needed to complete the compound.  For example, "cyanidin" (a polyatomic cation) can't be completely isolated by itself, but it could be incorporated into cyanidin chloride (a compound), which in principle could be isolated.