Heres a great image which explains cloaking in a little more detail:

What has Google Done?

It appears that the Google AdWords help centre has been serving up different pages to search engine spiders and users. Below is an example of the page users see vs. that seen by the Google crawler in its cache.

As a result of this, Google have banned these pages from appearing in search results. Performing a search for the exact URL currently shows no results in Googles index.

Search Engine Land asked a Google spokesperson to comment on the issue and they received the following response,

I can confirm that some Google support pages were inadvertently showing different content to the Google crawler than to users. This error has since been fixed. We will investigate how this happened and make sure that we take appropriate action.

It just goes to show that even the SEO police can sometimes get it wrong!