The other reply to your question (from guruevi) is correct. There's also the problem of "duty cycle" (there are other terms) which is basically how much noise is in the air on a channel. This is basically how much space is between the noise. When you have a crowded room with everyone trying to get online on a channel, they are all waiting for their turn to talk. The fewer people you have, naturally you have more time between transmissions. The more people you have, the less and less time there is between. It's like the difference between speaking slowly and those auctioneer guys who speed right along and are almost hard to understand because there's no space between their words to process what you're hearing. When you get a spectral analyzer out and look at the duty cycle on a channel in a crowded neighborhood (maybe an apartment complex), you'll see many access points (wireless routers usually) on each channel. If you look at say, channel 1, you have a duty cycle for the access points on that channel, and you have contribution to that from overlapping channels (2 and 3). So if you are using channel 3, you are contributing to the duty cycle (thus degrading performance and throughput) of channels 1 through 6. In return, those channels contribute to your noise as well.
The best practice is to keep the overlapping channels clean so that you don't get interrupted by those adjacent channels. The best possible graph you want to see in an analysis tool like inSSIDer on the 2.4 GHz view is 3 humps, spanning channels "0"-3, 4-9, and 10 to 13. In Europe you can use up to channel 14 as well. You'll get in trouble here in the US though. ;)
I'm not really familiar with posting things in /. elegantly, but here's an image of an ideal 2.4 GHz spectrum scan (you can use Wifi Analyzer on Android, or inSSIDer on Windows, or Wifi Explorer on Mac).
So here's the trick to using JUST 1,6, and 11. Signal strength. You can share channel 1 with as many neighbors as you want, but in order for it to be effective, you need to have roughly a 20 dBm advantage over the signale from the nearest AP using the same channel, in the area you want to use your AP. If you have that much of a difference in signal strength, it's good enough to have a good experience on wireless.
Here's an example where, if the signal from other APs are weak enough compared to your own, it doesn't matter that they are overlapping your channels because you can trump them with sufficient signal strength to talk from your client device to the AP.
So, if you have a scenario like that one, be courteous like the the taller ones and use the proper channels. :)
Here is an idea of what proper channel layout should be. If you have the opportunity to help your neighbors tweak their APs, I'd suggest channel planning it for the benefit of everyone. :)
I hope this helps. There is probably a lot more I could explain on this but I'm currently working a large event in Vegas and we're currently troubleshooting this very problem. Everyone comes in with a personal hotspot and just stomps all over the free wifi we are providing them. I wish there were a good way to educate people. It's just too technical for the lay person though.