The difference between the while and do while statement is that a do while will execute your code at least 1 time before it checks the condition. It will repeat (loop) as long as the condition returns True.
do {
# your code here
} while (condition)
Count to 20:
$i = 1
do {
Write-Host $i
$i++
} while ($i -le 20)
A shorter notation. Don't forget the semi-colon (;) delimiter here!
$i = 1
do {Write-Host $i;$i++}
while ($i -le 20)