How can a Human voice or animal voice have unique frequency Well this is pretty noobish question and I am not sure how to ask.
When We talk we don't talk in an uniform frequency. Then how can one measure frequency of ones sound/voice ? 
I am asking this cause several times I've heard people to say this is his/her frequency of voice. Is this a vague term ? or it has any real meaning ?
 A: Your voice, like any sound, is a combination of many frequencies.  
Physically, your voice consists of pressure waves.  If we plot the pressure as a function of time, we see that it goes up and down in a way that looks somewhat random.
You can measure these pressure waves with a microphone, then visualize them with an oscilloscope.  Here's a Youtube video where they do this, starting 4:50 into the video.
You may be able to do this at home using the microphone on your computer and some software like Audacity.
The data collected by your microphone is a time series.  The pressure is a function of time.
If you sang a pure note (or a reasonable approximation thereof), like you hear from an electronic tuner, the pressure would just be a sine wave.
You could imagine a more complicated sound that was two sine waves on top of each other.  This could produce beats.
As you add more and more frequencies, more and more complicated sounds become possible.
It is a remarkable result that in fact any sound can be represented as a sum of infinitely-many sines waves of different periods added up on top of each other.  This is Fourier's Theorem.
A human voice thus consists of many sine waves combined simultaneously.  Presumably, each individual voice has some special patterns to the way these frequencies are combined, assisting us in recognizing voices.  However, speaker recognition is probably based on other information as well.  I don't know too much about it, but you can check out the Wikipedia article.
We frequently try to isolate the different frequencies in a sound.  This is done electronically through electronic filters.  A crude example is "turning up the bass" - amplifying the low-frequency components of a sound.  Of course, a professional music studio has far more sophisticated control of the various frequencies.  This control can also be mimicked digitally through music sequencers.
On a cruder level, you could simply talk directly into an open piano.  The string in the piano will be excited by your voice.  The strings each have a specific frequency, so the strings that are excited the most tell you that their particular frequency is present the most in your voice.
Your ear accomplishes a similar task.  The cochlea has many small hairs, similar to piano strings, which are tuned to different frequencies.  When they vibrate, they mechanically trigger an ion channel to open, beginning an action potential that is eventually interpreted as sound by your brain.  So, in essence, you are distinguishing the various frequencies in people's voices already.
A: When people talk or sing, most sounds involve vibrations of the vocal folds in the voice box (larynx).
Like many musical instruments that produce sound by vibrating,
during any one short period of time a Fourier analysis will show some "fundamental" frequency (1st harmonic) which indicates how many times a second the vocal folds vibrate,
and also higher harmonics or near-harmonic frequencies.
(These higher frequencies help us distinguish different vowel sounds or different musical instruments, even when they play exactly the same fundamental frequency).
(A few sounds, such as the ones produced when people say the "s" and "t" sounds and a few musical instruments such as drum brushes, don't really have a fundamental frequency, but instead put out sound across a wide range of frequencies simultaneously).
It's pretty easy to measure the exact frequency produced when a person is singing a single note or saying a the vowel in the middle of a long syllable, using a spectrum analyzer.
As Mark Eichenlaub pointed out, several popular sound editing software tools include a spectrum analyzer.
Any one person speaking uses a range of frequencies, even during a single sentence.
Even so-called "monotonous" speakers have some variation in their fundamental frequency.
Each person has some range of frequencies they can produce.
"Monotonous" speakers use some narrow range of frequencies.
Singers, with practice, can sing a wide range of frequencies,
but it takes a lot of effort to produce sounds much higher and much lower than their normal talking voice.
Often choirs of people are separated according to the range of frequencies they can produce
-- voice classification.
Where I live, it is popular to divide choirs into 4 ranges:
soprano, alto, tenor, bass (in order from highest to lowest frequency).
(I hear that other regions divide choirs into more or fewer ranges and use other names).
Each range has a large overlap with the range above and below it.
The adult male humans in the bass section, because they have the largest voice boxes, can produce low frequencies that are difficult or impossible for singers in any other section to sing.
The female humans in the soprano section, because they have the smallest voice boxes, can produce high frequencies that are difficult or impossible for singers in any other section to sing.
The frequency range often varies from one person to another.
Soloists have some range of frequencies that they can easily sing loudly.
Often the instrumental part of a song is shifted up or down in frequency
to put the vocal part of that song into the range of today's soloist.
However, I wouldn't say human voice has a "unique frequency" -- given any one person, who has some frequency range, there are many other people who have practically the same frequency range.
It's similar to height. Given any one person, that person's height is usually in some fairly narrow range -- a range that varies significantly from one person to another.
But I wouldn't say a human has a "unique height" -- given any one person, there are many other people who have practically the same height.
