Well, I'm prepared to allow a professional voice coach explain it better than me, but the way I have understood it is simply:

"Normal" voice production produces a sound with the "fundamental" as what we take as the pitch of the produced tone.

"Falsetto" is called many things including "artifical" - the sound is not the fundamental, but an overtone of the fundamental of the "instrument" - in this case "the human voice". It need not be the first overtone, but you need to be good to control things - lots pf practice - I have never been able to produce more than one or two falsetto notes - but htne I have nver really tried as I do have quite an extended range - on a good night... singing wise that is ... :-)

On a related point, somewhere in my bottomless bag of tinwhistles, I have both

1) an Overton (or is a Chieftan?) "Overtone Whistle" (LowG fundamental) which has NO fingerholes, and is played by (over)blowing overtones & cupping the palm of one hand around the bottom hole and waggling it around like a leaf in a gale - visually not unlike a theremin... :-) by differently "loading" the exit hole with your hand, you can get a half octave, then you overblow again and get another half octave - a thorough b***ard of a thing to control, especially if you do have any muscular coordination hassles, but with effort, I can get a tune out of it - it sits unplayed for ages until I feel sufficiently masochistic to practice...

2) a normal D fundamental pitch whistle (looks like a Generation D!) with only three finger holes. Traditionally one of the holes is a thumb hole, but because of my micromotor hassle, I found it easier for me if all three holes were in the top as if they were the last three of a normal whistle. These were played in Medieval times (called a tabor pipe) with one hand and a tabor (drum) played by the other hand, either by hand or stick. You can also do this with a normal whistle by simply taping over or just not lifting your fingers off the top three holes - you can't get the bottom octave in full, but can achieve a full upper octave by utilising overblows. Normal Tabor pipes are twice as long as a standard pitch whistle, since you generate the useful notes an octave above what you would expect from the normal pipe. Because of this overtoning, the texture of the notes is slightly different - as is the falsetto human (male) voice - calling dogs would be probably be more appropriate for a female falsetto... :-)

You could if you wanted, call these two above musical instruments "falsetto" whistles.... :-)

Robin