What is the difference between an "Actively Scanned Channel" and a "Passively Scanned Channel"?

Actively scanning a channel means that our radio is allowed to send probe requests. Probe requests prompt AP's to reply – sending a "probe response". This allows our radio to see what AP's are available on each channel. Passively scanning means that the radio is not allowed to send packets, but only listen for beacons from the AP. Passive scanning is used primarily with DFS channels, where a radio must listen for an AP before sending probe requests.