How do google modules avoid spam?

some time ago i was using a joomla extension to manage forms for surveys.

The consequence was that my site was flooded by spam (my bad not using captcha). 开发者_如何学JAVA

So for this reasons and others I switched for an in-frame google module was forms. it was easier to create even by my non-tech collegues.

Of course with this method spam disappeared!

So i'm wondering, how is google able to filter the spam without using captcha or else?

Is due to the module being post-rendered in a frame that prevent bots from working, or google as a set of filter powerful enough to catch spam?

Thanks