While loops in ruby is not very different from while loops in other languages.

while _cond_
 _body_
end

So body runs only if cond is true.
For example:

i=10
while i>1
 print "We have " + i.to_s + " beer canz. -LETS PARTY HARD!!!- What should we do with them?"
 i -= 1
end

In my opinion, since we have iterators, while loops anre not very usefull.