Alaska's Senior Senator Ted Stevens was widely disparaged for a speech he gave back in June of 2006 when he compared the Internet as a "series of tubes". The backlash against the comment was quite remarkable, IMHO. People seemed to believe that this was an example of how stupid senators could be when dealing with technology (John Stewart did an awesome bit on it where he got John Hodgeman to admit "I'm a PC" on TDS (I can't find a link to the video unfortunately)).
The other day, I was chatting with a developer from the networking group, and I commented to him that I thought that Stevens was right. The Internet really IS a series of tubes. The other developer looked at me strangely until I explained my reasoning to him.
If I was to describe the whole "Net Neutrality" debate (a subject on which I am completely agnostic) to a layman, I think I'd end up describing it by using something like the "series of tubes" analogy (I'd probably talk about pipes and hoses though).
It turns out that when discussing bandwidth, pipes that carry water almost perfectly model the Internet. Bigger pipes can carry more water, smaller pipes can carry less water. There's no way of forcing more water through a pipe than the pipe can physically hold, and you simply aren't going to get 1Mb/s download rates through a 56kb/s modem. Similarly, pipes don't have to be filled to capacity - you don't have to use the bandwidth.
You could model the Internet as a series of interconnected pipes of varying diameters and length which carry data from the servers to the client computer, and not be too far wrong. Water (data) flows more quickly through the larger pipes and slower through the smaller pipes. Since the pipes are interconnected, data can travel via multiple paths between the server and the client, choosing the optimal path. If large sections of the pipes are blocked (water main breaks), the data is redirected via another path, potentially at a loss of bandwidth.
If you think about the Internet as a series of pipes, the "Net Neutrality" debate then boils down to a discussion about allowing paying customers to reserve space within the pipes (which reduces the capacity that the non paying customers have available) or equivalently allowing paying customers to use alternate pipes that are only available to those customers (thus allowing the companies to reduce the size of the pipe that's available for the non paying customers, since they don't use as much bandwidth as the paying customers). The people who are in favor of "Net Neutrality" want to force the pipe companies to keep the high volume traffic on the same pipes as their traffic (thus enabling more bandwidth for everyone), the people who oppose "Net Neutrality" (mostly the people who own the pipes) want to build an mechanism that allows them to gain additional revenue from the people who put the most data into their pipes.
If I had to guess, Stevens asked one of his staffers to explain the "Net Neutrality" issue to him, and the staffer came up with the "series of tubes" as a way of explaining it to Stevens, then Stevens ran with the idea, much to his later embarrassment.
 I am now officially flabbergasted. The wikipedia has an article explicitly about "a series of tubes". For the record, I wrote this post before I read it (since some of my comments above are reflected in the article).