What are Captchas?

"Because we care, we're security aware"

Whenever we create an account on any website or comment on any blog we see scrambled words
which we have to type and continue these are CAPTCHAs .  

What are CAPTCHAs and what they do?

It stands for Completely Automated Public Turing-test to tell Computers and Humans Apart.These tries to test that you are a person and not a auto filling software.

Why CAPTCHAs are necessary?

These are used to prevent hackers from avoiding misuse of online services.
There are some examples of activities which are undertaken by hackers are given below :-
-Opening someone's account  by attempting different  password again and again.
-Creating  hundreds of  free accounts-Swaying an online poll by robotically submitting hundreds of false responses.
-Spamimng blogs by posting dozens of bogus comments.
These CAPTCHAs test prevent about 90% of hackers attacks. 

What is a logic behind this CAPTCHAs?

 Commonly, these CAPTCHA phrases are .gif pictures of scrambled words, but can also be .mp3 voice recordings. These pictures and recordings are very hard for conventional software programs to understand, and hence, robots are usually unable to type the phrase in response to the picture or recording.