Captcha term explained
Captcha is a code that often you need to enter in forms when registering a new account on the services or add to the website any material by guests. The reduction originates from Completely Automated Public Turing test to tell Computers and Humans Apart.
SEO and captcha are not connected. When using automated tools, for example, for mass registration site in directories, hackers use OCR program decrypts captchas.
If you have a blog, it is advisable to add captcha to reduce the amount of spam and thus to avoid penalties from search engines.