On the old 11 Mbits/sec "original" Airport, aka 802.11b, there were 11 channels for data, and your Router could use any channel. For Single-band 54Mbits/sec 802.11g or 802.11n, there are now only three usable channels. Based on the old numbering system, these are channels 1, 6, and 11. Signal spills over into the adjacent two channels up and two channels down to fill the entire spectrum.
You can sometimes shoehorn your signal in by manually setting in-between channels, but results vary widely.
6 nearby Routers will eventually change channels until there are two on every useable channel, and that will fill the entire spectrum with competing stations, all trying to talk at about the same time.
A Dual-band Router (and dual-band Airport extreme card) can switch up to a different frequency band when things get crowded, and can operate on higher-numbered channels: 36, 40, 44, 48, 52, 56, 60, 64, 100, 104, 108, 112, 116, 120, 124, 128, 132, 136, 140, 149, 153, 157, 161, 165
This is what my report looks like, and I DO have an Apple Airport card installed: