[gentoo-user] Emerge question: What's with the @?

[Chris Lieb <chris.lieb@gmail.com>]

I've noticed lately on the ML that people have been talking about using
package sets, such as @world and @installed.  I figured it was a part of
portage 2.1.6* since using @world with 2.1.4* would result in an error
message about an invalid package atom.  However, after upgrading to
portage 2.1.6.4, I still get the same error when doing something like
'emerge -up @world'.

What are these package sets?  What is the difference between 'emerge -up
world' and 'emerge -up @world'?  Why don't these package sets ever work
for me?

Thanks,
Chris