I know some would say that laypeople should take the time to learn the technical, more accurate meaning of what the Internet is. The problem is that they won't. We all know laypeople. I live with two of them. When you start talking about "TCP/IP" or "DNS," or if you get far enough to start describing those terms, their eyes glaze over. That's what makes them laypeople — they don't care about the subject enough to learn about it in-depth; if they did, they'd be computer enthusiasts. So please keep in mind that, in order for this discussion to be useful, "succinct" and "layman's" are essential parts to any definition of the Internet given here. Also keep in mind that "succinct" doesn't necessarily mean one sentence; a relatively short paragraph would be fine, too — the main goal is to come up with something that physically describes the Internet which laypeople actually understand.
For those who don't know, Slashdot subscribers have been able to peek into the Slashdot submission queue for awhile in a neat feature known as the Firehose. I don't use the Firehose very often, but it's nice to have it there, and it's one reason I'm still a Slashdot subscriber.
So I peeked in today, and noticed that there's now a Firehose feed. Not sure if I'd ever want to subscribe, but that's pretty cool.