++i or i++ in for loops ??

++i is slightly more efficient due to its semantics:

++i;  // Fetch i, increment it, and return it
i++;  // Fetch i, copy it, increment i, return copy

For int-like indices, the efficiency gain is minimal (if any). For iterators and other heavier-weight objects, avoiding that copy can be a real win (particularly if the loop body doesn't contain much work).

As an example, consider the following loop using a theoretical BigInteger class providing arbitrary precision integers (and thus some sort of vector-like internals):

std::vector<BigInteger> vec;
for (BigInteger i = 0; i < 99999999L; i++) {
  vec.push_back(i);
}

That i++ operation includes copy construction (i.e. operator new, digit-by-digit copy) and destruction (operator delete) for a loop that won't do anything more than essentially make one more copy of the index object. Essentially you've doubled the work to be done (and increased memory fragmentation most likely) by simply using the postfix increment where prefix would have been sufficient.

As others have already noted, pre-increment is usually faster than post-increment for user-defined types. To understand why this is so, look at the typical code pattern to implement both operators:

Foo& operator++()
{
    some_member.increase();
    return *this;
}

Foo operator++(int dummy_parameter_indicating_postfix)
{
    Foo copy(*this);
    ++(*this);
    return copy;
}

As you can see, the prefix version simply modifies the object and returns it by reference.

The postfix version, on the other hand, must make a copy before the actual increment is performed, and then that copy is copied back to the caller by value. It is obvious from the source code that the postfix version must do more work, because it includes a call to the prefix version: ++(*this);

For built-in types, it does not make any difference as long as you discard the value, i.e. as long as you do not embed ++i or i++ in a larger expression such as a = ++i or b = i++.