What is the difference between ++i and i++

i = 1;
 j = ++i;
 (i is 2, j is 2)

i = 1;
 j = i++;
 (i is 2, j is 1)

x = i++ + ++i;

x = ++i + i++;

int i=100;
printf("%i;%i",i++,i);

for (int i = 0; i != X; ++i) ...