These work the same as do while loops only they execute until the condition evaluated is true
Here's a quick example:

$value=5;
do{
  print "$value\n";
  $value=$value-1;
} until($value<=10);

This is executes once and prints out $value;
5
Then $value is decremented and becomes 4
since 4<=10 is true execution of the loop is terminated

Now onto the exciting world of for loops