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);

1) Structure: The size in bytes is the sum total of size of 
all the elements in the structure, plus padding bytes.
2) Size of in bytes of the union is size of the largest 
variable element in the union. 

i.e In case of Union, the elements making up the 
union 'overlap' in memory OR they are accessed as diffrent 
name/type at diffrent places in the program.

Whereas in case of Struct, each of the elements have a 
distinct identity.

The difference between structure and union in c are: 1. 
union allocates the memory equal to the maximum memory 
required by the member of the union but structure allocates 
the memory equal to the total memory required by the 
members. 2. In union, one block is used by all the member 
of the union but in case of structure, each member have 
their own memory space

At the one time only one data member can be access...or can 
be used.

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.