What should one know about simple sugars?

Your question sounds simple but it's not.  Sugar of any kind provides energy - readily available for energy.  And sugar is not evil ACCEPT that we are eating too many processed foods with sugars that are "created" like HFCS - high fructose corn syrup,  the danger is that you are getting sugar without fiber or anything that can slow down its impact on your blood sugar levels - it get s"dumped in very quickly" and the excess calories - get stored as fat.  That is why we "preachers of health" are always saying - get your sweetnes from fruits and sweet veggies (like corn) becuase the sugar comes along with fiber.

So quite simply - simple sugar is 4 calories per gram and provides energy.