Sugar

Sugar is the generic named used to describe sweet-tasting compounds.
Sugar is both found in refined and processed forms (white sugar, cane sugar), and in various whole foods such as fruits.

Sugar and its different forms

The simple sugar forms are glucose, fructose and galactose. They're also called monosaccharides, because they're the most basic forms of sugar and cannot be further simplified.

Sucrose (white sugar), lactose (found in dairy milk), and maltose (found in grains) are disaccharides, because they're composed of two monosaccharides each--glucose + fructose, glucose + galactose, and glucose + glucose, respectively.

Glycemic index

Glycemic index (GI) is a value to represent how much a certain food will rise blood glucose levels, ranging from 0 to 100--with 100 being pure glucose.

Eating high GI foods makes the blood sugar level spike, and high spikes of blood sugar level lead to inflammation and a higher risk of cardiovascular disease by impairing endothelial function and thickening the blood (hypercoagulability). [1][2] This state is called postprandial dysmetabolism (a state of metabolic dysfunction for a period following a meal, which leads to higher inflammation and a higher risk of cardiovascular events [3]).
This is even more so important to pay attention to when it comes to people that are diabetic, and that are at a higher risk for cardiovascular events from postprandial high blood sugar levels. [4]
High blood sugar also leads to oxidative damage, since high blood sugar levels promote the production of free radicals. [5]

Having a spike in blood sugar levels also prompts the body to produce insulin rapidly, to keep the amount of blood sugar consistent. This then leads to a rapid drop in blood sugar levels, which leads to a drop in energy levels (also known as 'sugr crash').
Eating sugar also inhibits the orexin neurotransmitters, which stimulate you and give you a feeling of 'being awake'.[6] This, coupled with the insulin response, shows how eating significant amounts of sugar leads to a significant drop in energy after the initial 'kick'. This is why sugars are not an ideal energy source to keep yourself active throughout the day.

On top of that, eating high GI foods often can lead to a constant high level of glucose in the blood, which has been suspected to be a cause for type 2 diabetes, but this is still debated. [7]

However, it's a different story for fruits and the sugars eaten with them.
Not only because fibers counteract the inflammation by reducing it [8][9], but also because the antioxidants that the fruits contain fight the free radicals promoted by the high blood sugar level. [10]
By removing the fibers and turning the fruits into juices, the benefits of the fibers to lower inflammation are also removed. This, and the fact that more nutrients are removed by juicing, is why eating the whole fruits is recommended. That said, it's still preferable to eat fruits that are high in nutrition and low in sugars, both for weight gain and because eating fruits doesn't simply negate completely the risks associated with high blood sugar levels--meaning that the higher the sugar eaten with the fruits is, the worse it still is (to know what the best fruits to eat are, check the 'best fruit to eat' section).

In fact, whole foods that are plant based lower significantly the problems associated with postprandial high blood sugar levels. [11]

Weight gain

Refined sugars are very calorie dense, while offering no nutrients whatsoever (for the most commonly used ones). Similarly to oil, it's very easy to eat significant quantities of it and to not feel full because you didn't get any nutrients. Or if you feel full at the end of the day after eating lots of sugar, you'll have had less calories dedicated to nutrients, and more calories spent uselessly in empty calories (sugar), which will mean you're more likely to not meet the nutritional daily requirements.

Refined sugars are significantly higher in calorie density, since nutrients have been removed and only the calories of the sugar themselves are left.

Sugars with no nutrients:

• 1 cup corn syrup: 965 kcal
• 1 cup white sugar: 774 kcal
• 1 cup brown sugar (unpacked): 551 kcal

Sugars with nutrients:

• 1 cup date sugar (dried and pulverized, whole foods): 415 kcal with fibers, some vitamins and some minerals
• 1 cup maple syrup: 819 kcal with lots of vitamin B2 and manganese, and a bit of other minerals
• 1 cup blackstrap molasses: 960 kcal with optimal amounts of almost all minerals and vitamin B6, and some other vitamins
• 1 cup agave syrup: 1042 kcal with some vitamins

Sugars are best avoided to prevent weight gain, considering that they are very calorie dense while bringing no amount to very small amounts of nutrients (with the exception of blackstrap molasses and its mineral quantities).
A high BMI (obesity) leads to increase risk of all-cause mortality.