A sealed unit consists of minimum 2 panes of glass, separated by some form of spacer and then hermetically sealed. The variations are limitless. Glass Types & Thickness, Spacer Bar Size & Colour, Gas Filling these are all vital components in the construction of the sealed unit needed to achieve your requirements.
Dependant on the type of Spacer Bar used, it is filled by ratio with a product called desiccant which is a molecular sieve, similar to the silica gel, you find in the tiny bag in a, e.g. new shoes box. The purpose of this desiccant is to absorb any moisture in the cavity of the sealed unit after it is sealed.