Struct vs. Union

if you defined:
struct s
{
int i;
char c;
}

when you define a variable of type s, a memory of size equals to the size of integer + the size of character will be reserved.

In contrasts, when you define:
union u
{
int i;
char c;
}

A variable of type u, results in the reservation of memory size of the max of integer and character, and this makes you are able to use the type u as either integer OR character.

summary:
struct = AND
unio = OR