By default, packet captures are created every time you run Kismet. These are in wiretap format and by default end in .dump. This format is useful because you can open them in Wireshark for later deep analysis of the captured traffic.
Kismet has a unique feature that can cut down the time between wireless reconnaissance and wireless intrusion. During normal operation, whenever Kismet detects a data packet using a WEP initialization vector that is vulnerable to a related key attack, it stores these in a .weak logfile. These files can be used to speed up cracking wireless network encryption and are supported by most of the major WEP cracking tools.
Kismet also supports automatically decrypting network traffic for a network for which you already have the WEP encryption keys. This can be useful if you are monitoring your own wireless environment or if you have already cracked a network's WEP keys using another tool such as Aircrack (for more, see WEP and WPA Encryption).
To enable this feature, you need to edit your kismet.conf file and add a line like this:
wepkey=00:DE:AD:BE:EF:00,DEADC0DEDEADBEEFDEADF00D000000000000
This format is the BSSID (or MAC address) of the network followed by the hex representation of the WEP key.
By the way, at the time of this writing, Kismet is migrating to use an architecture currently labeled "the new core branch." This branch includes all of the same features as the current branch, but it has a much more intuitive user interface, including drop-down menus. The new architecture also makes it easier to add features and maintain the code. I expect this new branch to improve Kismet's usability significantly.