inSSIDer pulls data from both your WiFi adapter, and from Wi-Spy. You can use inSSIDer without a Wi-Spy, but it won't be nearly as cool.
WiFi data is pulled from your WiFi adapter on your computer. This can be a built-in WiFi adapter, or a USB WiFi adapter.
WiFi adapters can hear all of the networks around you, so we can tell what channels they are on, what their signal strengths are, what their security settings are, and more.
What we don't get to see is how busy each channel really is, nor do we get to see interference from non-WiFi devices.
Spectrum data is pulled from Wi-Spy, which is a special device called a "spectrum analyzer." Unlike a WiFi adapter, it can see all of the raw radio frequency activity around you.
This allows you to see how busy each channel is, as well as interference from non-WiFi devices.
In this example, we can see that there is a non-WiFi device on channel 1, and that there is a tiny bit of Wifi activity on channel 11.
When you have both WiFi data and spectrum analysis data, inSSIDer overlays them so you can compare.
With both WiFi data and spectrum analysis data, we can see that the non-Wifi device on channel 1 isn't interfering with "MetaGeek Lab" on channel 11.