The public key you must get from Google in order to use the Google reCaptca web-service.
The private key you must get from Google in order to use the Google reCaptca web-service
You choose between 4 different skin
The tab index of the text field where the user must enter the validation texts
The language you want the object translate the texts
The other fields are the texts displaied in the captcha object and when you choose Custom language you can customize all this texts. Like you can see in this image: