What is the difference between monatomic and polyatomic ions?

${H}^{-}$, ${X}^{-}$, ($X$ = halogen), ${O}^{2 -}$, are all examples of mono-atomic ions. Polyatomic ions include sulfates, $S {O}_{4}^{2 -}$, nitrates, $N {O}_{3}^{-}$, thiosulfates, ${S}_{2} {O}_{3}^{2 -}$.