The Whistle is an important way for a registered user to get involved with Newgrounds. It enables the user to report abuse. The colour of the Whistle indicates its power and is judged by Stat Enthusiasts/Stat whores as one of the most important stats of Newgrounds users. Its level is determined by whistle points, which can be positive or negative.
The whistle is the representation of the user's power to report abuse of the Newgrounds systems, like reviews or stolen flash. The higher the level, the higher the impact on the review/flash is. A garbage whistle won't have any kind of impact, but using it can still get points, which may take the user back to an effective whistle.
2013 Redesign Levels:Edit
The only way to gain or lose whistle points is to flag reviews or flashes. Flagging a review or a flash will award some points if the material is deleted, but if flagged incorrectly the penalization will be greater.
Stat Whores take advantage of this by searching for and flagging blatantly abusive reviews, like the "you suck" kind. After they get the deity whistle, they may not bother to flag any more.
The amount of whistle points is hidden, and only administrators know the exact amount of whistle points someone has.
Further Info Edit
The Expert's Guide has information about Whistles and Whistling including a quiz on the subject. The current most known helpful guide other than this for Whistle Status in SuperMarioBrosS: Whistle Stats Page