I being a chat site owner for the past 3 years beleive the difference between a chat room and a virtual world is this...
Virtual world's give you the opportunity to make a character like you can control there personality... so its basically like throwing yourself into a world online, some people choose to not be them and live a different world but in a way makes it feel more "real" and even can be like a fantasy
Now depending on the type of chat room... for instance a basic text chat kinda gives you some security you can hide behind your words if you choose too.. its not as personal unless you put your business out in chat. Now my chat on the other hand is a Cam/Voice chat so its extremily personal and really gives the opportunity to find friends and or relationships and makes it even more real then a Virtual World...but it also has the option to not have cam or Mic and just type which then gives some people that security.
So thats my perspective... in alot of ways there the same thing and all depends on the type of virtual world or chat...I personally like the Cam & Mic because it gives me the chance to meet people and see them live all around the world and hear there accents etc...