That's an expression used by youngsters on an on-line bulletin board called 2 ちゃんねる (2 Channel).
ﾜﾛｽ (warosu) derives from 笑う(warau, to laugh).
Just think of ﾜﾛｽです as an equivalent of "lol" or "lolz". However, unlike "lol", it is ONLY used by kids on 2 Channel and on a lot of their blogs. I suppose it is becoming more common among young people in general.
Don't use it unless you want to sound stupid. Certainly not to be attempted by someone who is studying Japanese. In my opinion, anyway.