What is the difference between "active" and "passive" sniffing?

Active Sniffing Passive Sniffing
* In active mode the WiFi device sends out probe request and waits for answers. The data obtained from these answers is shown. * When you use KisMAC passively the WiFi device is switched into "monitor mode". It waits silently for any valid packets and this data is used to derive the network structure.
- Probe requests can be easily detected, even by people using KisMAC in passive mode. + Passive sniffers cannot be detected
- The sniffer cannot see raw data, therefore PCAP logging and cracking will not work. + Only passive devices can perform WEP cracking and logging of data.
* WPA networks can be found in active mode now. * WPA and LEAP detection is possible
- Active programs cannot see hidden networks (per definition). + KisMAC can find and reveal hidden networks if used in monitor mode
+ You will not lose your WiFi connectivity (it will only become slightly slower because of all the probing) - KisMAC will need to load a replacement for your normal driver in order to switch the device into monitor mode. Connectivity will be lost.
+ Airport Extreme is supported. + Airport Extreme cards are now supported as well.

Basically: If you have an active mode driver,  you will be able to find open, WEPed and WPAed networks. You will not be able to crack these networks or find closed networks.

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