I suppose one way you can think of a union is that it is a set of aliases of varying type to a block of memory where each member of the union is an "alias" with a given type. Each alias refers to the same address in memory. How the bits at that address are interpreted are determined by the alias' type.
The amount of memory the union occupies is always equal to or possibly larger than the largest sized "member" of the union (due to alignment restrictions).
