The key difference between polyatomic ions and compounds is that the polyatomic ions have either a positive or negative electrical charge whereas the compounds have no net electrical charge.
A polyatomic ion is a term that we use to name chemical species containing two or more atoms that have a net negative or positive electrical charge. The electrical charge of this ion is a result of the number of electrons present in each atom; if there are more electrons than the total number of protons in atoms, it gets a net negative charge and vice versa. Compounds, on the other hand, are chemical species with no electrical charge. They have equal numbers of electrons and protons.
