Let's nitpick some of the terms used in this context.

Virtual.

The way I interpret the meanings laid out in my dictionary, it means:

1. Real, but not really.
2. Illusionary.
3. Simulated.

The first one tells us that reality depends on perspective, the second insists on widening the gap, and the third one is actually useful but not in this context.

All in all, 'virtual' is not too clear a word, but I'll keep using it because of the lack of good alternatives and because I like the implied meaning that reality is in the eye of beholder.

Cyber.

Anything to do with computers, computer programs, or a network of these.

Not a very fancy definition, but a useful one.

Cyberspace.

Human interactions and relations as expressed through cyber.

Since we like to look through the lenses of space-time continuum, we experience the possibilities and choices computer technology provides as breathing space.

Cybersphere.

I made it up. (At least the intended meaning.)

One in the line of similar -sphere words (eg. blogosphere), cybersphere is halfway between cyberspace and noosphere.

I think of it as 'applied' cyberspace; signifying the cyber level of reality as integral to our 'real' reality.

Virtual reality.

As two words slapped together, it's only a meaningless paradox.

Usually in this context it means a computer simulated 3d environment. Because it's originally meaningless, we can sweep plenty of other meanings under it.

World.

A reasonably complex stand-alone set of things.

(Not the best of definitions, but it can be good enough in this context.)

Virtual world.

A world in the cybersphere, sustained by soft cyberspace and hard cyber - with an interface also referred to as 'virtual reality'.