Spam can be annoying, especially when you get thousands a day. By adding a captcha to your webforms you can greatly reduce the amount of spam.
There are many free options available but you usually have to make an account with the captcha provider. Some captcha options are better than others because they are more frequently updated. Spambots are always learning of the new captcha techniques so the better maintained and updated the captcha is the more effective it will be.
What adding a captcha usually does is add a verification at the end of your webforms where the user enters a specified phrase, or checks a box to prove they are human and not a spambot.
Interested in adding a captcha to your site? Check out www.mollom.com.