We use reCAPTCHA (No CAPTCHA reCAPTCHA by Google) – A CAPTCHA-like system designed to establish that a computer user is human (in order to protect signups from scripts and bots).
Using reCAPTCHA allows us to make the signup flow as easy as possible.
reCAPTCHA Terms of Service – https://www.google.com/intl/en/policies/terms
reCAPTCHA Privacy Policy – https://www.google.com/intl/en/policies/privacy