Do you think there is a difference between a virtual world and a MMORPG?Yes, I think so, though I'm a bit hesitant in my answer, not having really ever experienced MMORPGs. I know of WoW, and I might be able to come up with a name of another, but that's not too promising. On the other hand I can name a fair few virtual worlds and synthetic environments.
My first reason for differentiating is based on the role playing aspect. I am assuming that the RPG of the MMORPG title indicates that you would always be role playing. Conversely I'd suggest that while you might role play in a virtual world, it's not a given - you might just as well be 'playing' yourself. If there are MMORPGs where you play yourself my argument would get fuzzy... I look forward to learning more!
I'd also look at the use of the term 'game' in the MMORPG title. Do games by necessity have aims and rules? If so, I'd say that virtual worlds differ in that (while users in them in a particular context might have or impose aims or rules) they generally promote more free exploration, without any particular set of objectives. Users might enter and user a virtual world for a whole range of reasons, and these may or may not include gaming.