What is the concept of padding?

Can any one tell what is the concept of padding in c and c++.

and how much space following user defined variable will take:

Code:

struct num
{
int n;
char ch;
int p;
}

struct num
{
int n;
int p;
char ch;
}

struct num
{
char ch;
int n;
int p;
}

also tell me how the concept of padding is applying in these cases.