Numbers to Register

[Bahco]

How do those numbers work i.e. to register on certain sites like Yahoo or Paypal you have to key in numbers or letters to get registered.

Do these numbers confirm your IP address or is it something else?

 

[MarkBarl]

I think what you are referring to is one of these

It is called a captcha. It is a security method designed to verify that a human is at the keyboard and not an automated program. The text is easy for a human to decipher but difficult for an automated system. A couple of sites with more information are
http://en.wikipedia.org/wiki/Captcha
and
http://www.captcha.net/

Recently there was an attempt for spammers to get people to help them out unwittingly by sending email of women in various states of undress along with a captcha. you entered the text in the captcha and then you got another picture with more clothes taken off. Full story here

 

[Bahco]

Good answer Mark thanks.