A certain amount of sugar is actually necessary for health, but not much. You should not have more than 70g of added sugar in your diet per day, or 50g for women, and this includes fruit juice and natural sweeteners like honey. If you eat a lot of fruit you should also take less added sugar; so the amount of actual sugar you should put in your food is very small.
Refined white sugar is the worst for health, as it is high in calories with few nutritional benefits. The main problems with eating lots of sugar are mood and energy swings in the short term, dental decay in the longer term and, most of all, being overweight leading to heart and other serious illnesses.
Sugar doesn't satisfy, so if you eat something sweet you will soon be hungry again; so you have the calories from the sugar as well as your next snack. It is also addictive, so the more you eat the more you want.