A "Foof" or a "phoof" is the unofficial name for the black and white "preview" at the beginning of each act of an NCIS episode.
It made its debut appearance in the NCIS Season 2 episode, Lt. Jane Doe (episode) and as such, has aired in every single episode of NCIS to date except for NCIS Season 1 and the first three episodes of NCIS Season 2, See No Evil (episode), The Good Wives Club (episode) and Vanished (episode) of Season Two.
As such, each episode has had a total of four or five foofs with the majority of the foofs including the main characters.
The "foof" gives NCIS the show something of a very unique quality and separating it from other shows including NCIS's own spin-off series, NCIS: Los Angeles.
NCIS's creator and Executive Producer Donald P. Bellisario creates the noise by hitting a microphone with his hand although it sounds like a light-bulb exploding.
In the Season 3 debut episode, Kill Ari Part 1 (episode), and unlike in previous episodes which had the characters moving to some degree even after the foof itself happened, the foofs from "Kill Ari Part 1" began using freeze-frames which has continued to be used to this day which means that every time a foof happens, then the foof itself will end with the people standing still like a photograph of some sort.
When the Season 5 premiere episode, Bury Your Dead (episode) aired, the foof was again changed so that it would end on a different one compared to the one originally shown in the first "preview".
In the Season 5 episode, Leap of Faith (episode), the three-second teaser being shown at the beginning was changed so that it was NOT for the end scene before the intro (like in past episodes) but it was actually for the scene after the intro leading into the first commercial break.
Foofs are mostly in black and white - but not always completely so due to the fact that at the end of the Season 7 episode Code of Conduct (episode), Anthony DiNozzo inadvertently displayed a flash of color. Having taken a sip of 'coffee' that Ziva David had bought him, he smiled, revealing that she'd played a Hallowe'en prank on him - his mouth had been stained blue. When the foof occurred, Tony's teeth remained blue while everything else in shot turned monochrome.
This happened again in the Season 11 episode, Homesick (episode) as the final foof showed Gibbs and the entire squadroom shrouded in the black and white freeze frame while the Christmas tree was left untouched.