WSJ has this interesting analogy:
Think of information as water. A library, therefore, is a lake. The information is just poured in there, as books and periodicals. Those who want to use it wander in and scoop the water out. There’s water coming in and going out, but most of it just sits there: still water, that we have to go to in order to enjoy it.
Web pages are much the same. Information is added to the lake that already exists, but for the most part it’s a pretty static, if not stagnant affair.
Email is different. There the water comes to us in buckets. Much more useful, because the water is no longer stagnant, and we don’t have to go and scoop it out ourselves. But we are still dependent on someone sending the stuff to us — filling the buckets, as it were — and we also have little control over when, how and what kind of information we receive. No surprise, then, that one of the shortcomings of email is that we find ourselves receiving lots of waste water — spam — along with the potable stuff.
If information is water, surely there must be a way to pipe to our house just the kind of water we need, when and where we want it? This is RSS: a way to deliver information to us in a way that suits us. RSS is the piping and the faucets that let us order and manage that information flow.