In the left hand column (the Tag Browser windows), click on the dropdown arrow by Tags. This should show you a list of what tags exist in your library. Some of them may be genre related, others can be almost anything including such popular bits of uselessness such as "novel", other authors, etc.
Personally, I used to use a collection of custom columns to generate collections (genre, finished, etc.) but the only survivor is the finished collection. I ended with too many collections for them to be useful.
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