Serums normally contain higher concentration of active ingredients and are thicker in consistency.
Toners and essences are more marketing terms because they can be anything - toners are normally more runny and essences are slightly thicker.
It's best to select products by their ingredients and not by how they've been marketed.
If you had all three in your routine, you'd apply toner - > essence - > serum (thinnest to thickest), but none of them are essential. I personally only use serums.