There are few form controls of CAPTCHA type. Navigate to Site manager -> Development -> Form controls and filter out CAPTCHA controls. You'll find e.g. "logic captcha" or "text captcha". You can use those controls e.g. in forms (this is typical usage) or anywhere else (just see properties of particular control).
If you want to prevent robots posting to your forms edit the form, add a text field and select CAPTCHA of your choice as a form control.