The googleon/googleoff tags are index, anchor, snippet, all. Here's how they are used:

index tag

Words surrounded by the googleon/off tags will not be indexed as occurring on the current page

A page containing:
fish <!--googleoff: index--> shark <!--googleon: index--> mackerel

has the terms "fish" & "mackerel" indexed for that page, but will not index "shark" for the page. It's possible, however, that the page could be a search result for the search term "shark", since "shark" may occur elsewhere on the page, or in anchortext for links to the page.
anchor tag

"Anchortext" surrounded by the googleon/off tags and occurring in links to other pages will not be indexed as words associated with the other linked-to pages. A page containing:
<!--googleoff: anchor--> <a href="linked_to_page.html"> shark </a> <!--googleon: anchor-->

will not cause the word "shark" to be associated with the page "linked_to_page.html". Otherwise, this hyperlink could cause the page "linked_to_page.html" to be a search result for the search term "shark".
snippet tag

The text surrounded by googleon/off tags will not be used to create snippets for search results.
<!--googleoff: snippet--> come to the fair! <!--googleon: snippet-->
all tag

Turns on all of the attributes: index, anchor, and snippet:
<!--googleoff: all--> come to the fair! <!--googleon: all>

The text surrounded by googleon/off tags will not be indexed, followed to another linked-to page, or used for a snippet.