A 'virtual package' is a package that does not install any files itself, but instead is dependent on some related 'set' of packages. That is, when you install it, apt-get (or aptitude) installs a group of packages. It is a convience hack. Rather than tell people:
apt-get install foo bar baz whatever mumble liba libb libq
you can instead tell people:
apt-get install interesting-package
'interesting-pacakge' lists foo, bar, baz, whatever, mumble, liba, libb, and libq as packages it depends on.