i++;

simply mean

i=i+1;

i++; will just increament value of i by 1.

in c,or c++

i++,++i are same if. if they are not used with other variable.

in the same way --i,i-- are same.

consider

i=5;

i++;

now the value of i is simply 6;

i=5

++i;

again value of i is 6;

but u just consider statement

i=5;

c=i++;

the value of c is 5, i is 6.

i=5;

c=++i;

the value of c is 6,i is 6;