You may have seen many time this captcha in online registration forms , login pages and where you have to retrieve your password. You asked to enter some words those are written in some difficult form to check you are human.
Some website asked to write words some wants to do some simple math like 2+6= etc. You may wonder what is this captcha and why its is needed.
As per wikipedia ” A CAPTCHA or Captcha is a type of challenge-response test used in computing to ensure that the response is not generated by a computer”
Internet is full of people those are only there to make trouble for others like hackers or miscreants. Those job is to put every system or service in trouble using automated user id, password generated to send thousands of new email or new accounts requests to that server will go down to handle these fake requests.
CAPTCHAs are used in attempts to prevent automated software (bots) from performing actions which degrade the quality of service of a given system, whether due to abuse or resource expenditure. CAPTCHAs can be deployed to protect systems vulnerable to e-mail spam, such as the webmail services of Gmail, Hotmail, and Yahoo! Mail.
In captcha words shown on screen are matched with the words you type with already saved letter or phrases, in some websites when words are too long like 12-15 characters than I have found that if one or two letters are wrong they are taken avoid users problem to type again and again.