All the members of the structure can be accessed at
once,where as in an union only one member can be used at a time.
Another important difference is in the size allocated to a
structure and an union.
for eg:
struct example
{
  int integer;
  float floating_numbers;
}
the size allocated here is sizeof(int)+sizeof(float);
where as in an union
union example
{
  int integer;
  float floating_numbers;
}
size allocated is the size of the highest member.
so size is=sizeof(float);