well In structure you can think up like a class that when an object is created

it consist of all the data members declared in it .

say class A{

int a;

float b;

char c;

}

so when you create an object say A obj= new A();

it will occupy 2 bytes for int +4 bytes for float+1 byte for char so in total 8 bytes

where as in Union it will occupy the bytes of the 

highest data type say here is float so it will occupy only 4 bytes.

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A good example is when you have a data file with a fixed-size record,
but the record may contain different things. For instance,
one record may contain 64 characters of text.
Another record may contain 16 4-byte integers.
You may then have something like

typedef struct 
{ 
    int record_type; 
    union 
    { 
        char text[64]; 
        int values[16]; 
    } 
} RECORD; 

In this example, the struct member record_type can be used to determine
if the data are to be read as a text string or as an array of integer values.