what are internal padding

in my code i have three classes
class A
{
protected:
int x;
};
class B:public A
{
protected :
int y;
};

class C:public B
{
protected:
int z;
};

in the constructors of each class i cout the sizeof(class)
on console the size is 4 bytes of each class...but as soon i include a char member ...the size of the respective class increases by 4 bytes...though the sizeof(char) is 1 byte.
In help i got it may be due to internal or trailing padding.What are this padding and whats their significance....pls explain me.

padding means adding extra bytes to make things seem in order. it cuts time at the expense of space.