In simple terms, the more people share a rivalrous good, the less there is for anyone; when they share a non-rivalrous good, everyone gets the same amount, and the amount doesn't decrease with more users. Apples and seats on a plane are rivalrous; an idea, radio broadcasts and the common cold are non-rivalrous.
Goods are exclusive if producers can prevent people from consuming them if they haven't paid. I can prevent someone from using an exclusive good, but I can't prevent someone from passing on a non-exclusive good like an idea. It pretty easy to make tangible goods exclusive; I can lock up the apples. It's harder with intangibles, as the struggles over managing access to digital music have illustrated.
Some goods are both non-rival and non-excludable, like national defense, and non-DRM protected digital music. Public goods are non-rival and non-excludable. The rival/non-rival and excludable/non-excludable concepts can be shown as a 2x2 matrix:
|Rival||Tangible private goods (eg airline seats)||Commons (eg unlicensed spectrum)|
|Non-rival||Intangible private goods (eg patented inventions)||Public goods (eg national defence)|
These definitions, however, don't take into effect the network effects that have become so prevalent on the web. Social networks like amazon reviews and del.icio.us tags are not just non-rivalrous, as one would expect from knowledge; the more one uses them, the more value is created.
These goods are "anti-rivalrous". Their use increases the amount available for consumption by others.
One can play the same game with exclusiveness. An "anti-exclusive good" might be one where the my giving it to you actively encourages you to pass it along to others. Viruses are one example; another is peer-to-peer software which someone cannot use without becoming a server node for others.
Expanding the table above to a 3x3 gives:
|Anti-rival||(eg member-limited social networks)||(eg communal site tagging)||(eg P2P supernodes)|
Note that I haven't given names to the cells - that's left as an exercise to the economists...