“CAPTCHA”, which is trademarked the Carnegie Mellon University, is a short form for “Completely Automated Public Turing test to tell Computers and Humans Apart”. It is a test, which helps the computer to differentiate between machines and human beings. It has the capability of creating and grading tests that automatically accepts all humans and rejects all machines.
It works on the assumption that humans can read distorted machines but machines usually cannot do so. You can know more about CAPTCHA and smart CAPTCHA at http://www.protectwebform.com/ and http://www.protectwebform.com/smartcaptcha.
Most website owners find CAPTCHA very useful. Such tests help to prevent programs known as bots. Bots are programs that pretend to be humans and abuse the services of the internet. Bots try to sell or make you sign up for thousands of free mail services. The bots spam your inbox daily. They intrude into common chat rooms to accumulate personal information and publishing links to various sites for promotional benefits.
In addition these bots can put a worm on your system, getting your passwords, and other personal information and taking it back to the Internet for others to use commercially without you even knowing how they got it.
You can use a CAPTCHAS or a CAPTCHA plug-in (http://www.protectwebform.com/plugin_wordpress) to stop these bots from taking over your system. Many sites of all sizes have installed these. Once the CAPTCHA is running as bots try to get into your system, they will be asked to type out a distorted image shown on the screen. Since computers can rarely translate these messy images into letters as humans can, it can stop the bot. you may have seen a CAPTCHA when logging onto your mail service, into a chatroom, or onto your bank website.
Spammers send unsolicited messages in bulk to you inbox. These messages are not desirable at all. The most common kind of spam is email spam. However there can be other spam too in the form of usenet newsgroup spam, log spam, search engine spam, instamessaging spam and similar others.
Advertisers resort to spamming as spamming does not incur any kind of operating costs other than the cost of managing the list of individuals that are targeted to receive the mails. Also you cannot hold the sender accountable for the mass mailers they send. Since there is no entry barriers, spamming has become really ubiquitous. This is the reason why all your inbox is so inundated with mass mailers whenever you access them.
Usenet spam is a message that can be sent to twenty or more Usenet newsgroups. This kind of spam is usually targeted to those people who visit and read the newsgroups but do not post anything or give any kind of address. This kind of spam inundates the newsgroups with unnecessary advertisement and posts. It also challenges the owners ability to accept and manage the topics on the system.
The other kind of spam, which is the email spam usually targets individuals with direct messages. The list of ids that are targeted to spam can be created by scanning postings from Usenet or pilfering with the Internet mailing list. This kind of spamming does incur a little more money.