The Sara structure is a memory block containing the variables inside. There is nearly no difference between a classic declarations :
char first[20];
int age;
and a structure :
struct Person{
char first[20];
int age;
};
In both case, you are just allocating some memory to store variables, and in both case there will be 20+4 bytes reserved. In your case, Sara is just a memory block of 2x20 bytes.
The only difference is that with a structure, the memory is allocated as a single block, so if you take the starting address of Sara and jump 20 bytes, you'll find the "last" variable. This can be useful sometimes.
check http://publications.gbdirect.co.uk/c_book/chapter6/structures.html for more :) .