A long long time ago, way way back when some people still had 8-track tape players, there was The Phone Company. Except for a couple little insignificant private companies (like GTE and Continental) all of that wire belonged to Ma Bell. It was pretty easy to picture how your voice got from Miami to Madison because this network was pretty much one-dimensional.

The Internet, today, is not "owned" by any one company. It originated as a private academic/military network with a single provider connecting all the users together but became a public network-of-networks with multiple providers. If you're a capitalist (or an evolutionist) you like diversity because it provides choices and engenders quality and advancement.

But What's It Look Like?
The Internet is composed of major networks (providers like sprint, uunet, verizon, qwest, level3, etc) called Autonomous Systems or "backbones".  They connect the local ISPs (known as "last mile" providers, like your DSL provider and my cable company) to the rest of the world. They might be considered freeways while our last mile providers would be surface streets. Try to picture a spider web in 3D. Now picture a couple more of them above and below each other. A spider can go all over his web but can't get to another one. So they have places called peering points where these webs are connected to each other. Now, if my cable company uses LEVEL3 as their backbone provider and yours uses AT&T, my packets go from my house, thru TimeWarner cable's net to LEVEL3's backbone, to a peering point, to AT&T's backbone, to your ISP to you. Here's a very cool map, from CIO.com, of the NA internet - it's not geographic, it
just shows connections and who the main backbone players are.

See For Yourself
You can actually see the path that data packets take from you to any web site, mail server, etc. Kinda like an Internet GPS "make the next turn at Verio Router, LAX. Now make a right at your destination, Yahoo.com"  Using Windows, from a command prompt, type:  TRACERT  AnyWebAddress  <enter> (in MacOS use Network Utility > Traceroute; for Linux/Unix, see here)   You will slowly get a collection of steps showing you each router "hop" between your end and the destination. Conventionally, engineers name the routers using nearby airport codes. So a router in the LA area would include LAX in the name and one in San Diego would incl SAN and Bogota would include BOG. It's interesting to see the geographic path that your packets take to get someplace and, if there's an internet outage along the way, you can see where the packets stop.

The fact that there are multiple networks, and the fact that there can be routers and fiber out of commish anyplace at any time (and the beauty of IP is that it allows routers to report to each other about best routes - like traffic reports) explains why the "best" route from you to your favorite web site might not be the geographically best route.

Here is a good, basic, overview of what "the Internet" is, without all the tech talk. To get an idea how much it's changed since 2000, go here and, starting from the bottom of the page, see the historical charts.

Net Neutrality
The CIO.com article mentioned above is about an important subject, one dear to the hearts of the folks at newmediarights.com and UCAN.ORG, and that is about keeping the internet free of content restrictions. If there were only one backbone provider, they could say "we will block the transmission of (e.g.) music files because we also sell music on our web site and ones you buy elsewhere thus compete with us". They could be less protectionist and more protective of themselves from litigation: "we will block the transmission of (e.g.) music files because they may be copied and we could be sued if they go across our network". And they could be interested in filtering your 1st Amendment-protected free speech because they don't agree with you. These examples are pretty black-and-white but there are other, less clear issues currently in play. For example, some networks won't allow people to use mass-dowload applications, like BitTorrent, because, they believe, these people are using more than their fair share of the bandwidth available. I can see where I, personally, might not want to have my email be delayed or have my rates go up because some people need to download large files (full length movies, for example). I can also see where the provider might want to block something I want to download. Some balancing has to happen.

Net Redundancy
The main reason there's more than one backbone provider is, simply, redundancy. Having all your bits in one basket isn't a good idea. And while, not so long ago, The Internet was a toy for amusement and non-realtime academic interchange, today a lot of people rely on it, now, this instant. Having more than one means that your packets have options.