Sugar is a class of sweet-flavored substances used as food, a form of carbohydrates. Among the types of sugar (or monosaccharides and disaccharides) there are glucose (simple sugar), lactose (in milk), fructose (in fruit and honey) and sucrose (sugar coming from beet and cane).
In cooking sugar is an essential ingredient. There is no dessert, cake or cookie with no sugar at all. But it is also used to make jame and marmelade, fruit in syrup, liqueurs and preserves, and to sweeten beverages, like tea and coffee. Beside granulated sugar, you can also use milled (icing) sugar, both to make and decorate cakes. Invert sugar, a mixture of glucose and fructose, is also used, as it is sweeter than usual sucrose and less prone to crystallization.
Sugar can be an important source for food energy, but since the twentieth century, it has been certified that a diet high in sugars, especially refined sugars, may bring to obesity (and, therefore, to diabetes as well), tooth decay and cardiovascular diseases. Besides that, according to a number of studies, consumption of sugar (or sweets) could have an addiction effect.
Anyway, there are several alternative to sucrose, less harmful for human metabolism, like honey and fructose, or syrups coming from some plants, such as maple syrup.
Fun fact: Sugar has been known in Asia since ancient times, but honey was a much cheaper alternative. This till the diffusion os sugarcane plantation, in the XVIII century.