Aerogels are fascinating materials. Give a piece a silica aerogel into someone's hands, and that person is immediately fascinated and curious about how such a solid, stiff material can be that light and transparent able to withstand a burning flame of a welding torch, and that they can still hold it in their hands without any feeling of it becoming hot. They feel the same experience if they hold, for instance, a cellulose aerogel in their hands: it is equally stiff, light, white and feels like a marshmallow without having a glueing touch to the fingers. The introduction discusses the understanding and various definitions of aerogels and classifies the types of aerogels developed so far. Finally, the chapter gives a brief overview of the history of aerogels.