The quicksort is considered to be very efficient, with its “divide and conquer” algorithm. This sort starts by dividing the original array into two sections (partitions) based upon the value of the first item in the array. Since our example sorts into descending order, the first section will contain all the elements with values greater than the first item. The second section will contain elements with values less than (or equal to) the first element. It is possible for the first element to end up in either partition.
An example in C

void swap(int *a, int *b)
{
  int t=*a; *a=*b; *b=t;
}
void sort(int arr[], int beg, int end)
{
  if (end > beg + 1)
  {
    int piv = arr[beg], l = beg + 1, r = end;
    while (l < r)
    {
      if (arr[l] <= piv)
        l++;
      else
        swap(&arr[l], &arr[--r]);
    }
    swap(&arr[--l], &arr[beg]);
    sort(arr, beg, l);
    sort(arr, r, end);
  }
}

Exernal Links:
An interactive demo; http://pages.stern.nyu.edu/~panos/java/Quicksort/
Wiki; http://en.wikipedia.org/wiki/Quicksort