i don't understand about prefix and postfix..

Answer 1

see prefix will increament before ending of code but postfix will increament after the code is ended
eg) void main()
{
int a=10,b,c;
b=++a;
c=a++;
cout<<" value of b is"<<b<<endl;
cout<<"value of c is"<<c<<endl;
cout<<"value of a is"<<a<<endl;
}

output
value of b is11
value of c is11
value of a is12

here the value of b is 11 becoz ++a will increament before ending of semi colon but the value of c remains 11 becoz a++ increament value will reflect after ending of code i.e after the semi colon so the final value of a is 12

Answer 2

prefix is first increse then use and postfix is first use then increase

Answer 3

About performance, if you dont care which one to use, for example, in a statement like this:
x++;
You should always use the prefix notation ( ++x ) instead of the postfix ( x++ ) notation because the prefix increments the value and it uses it, while in the postfix notation first a copy of the object is made, then the original value is incremented, then the copy is used and then destroyed.