The Genre Attribute is predominentaly for the "Media" industry, such as music, literature, and movies.

For example in Music it would be entries such as Rock and Roll, Pop indie, Rap, Etc

In movies it would be Horror, Comedy, Drama etc

Here is a list of Genres you could use
http://en.wikipedia.org/wiki/List_of_genres
or
http://itunes.apple.com/uk/browse/
Select from the categories at the top, for Music, Movies etc.

It appears that google themselves don't really know what to do with this attribute, as i have asked specific questions about its use and had very general replies back
http://www.google.com/support/forum/p/base/thread?tid=7d788158a71ea17c

So you can make of this what you wish lol.