In formal writing, the Em Dash (the type of dash being used here) is used to indicate a change in focus or structure within a sentence. There may be a short digression within a sentence:
"Yesterday, Sally—man, I hate that girl—told me my hair was unkempt."
In this sort of case, two dashes are used: one at the beginning of the digression and one at the end of it. There are also cases where the sentence never quite gets back on track:
"I was at the Bargain Mart when—don't you know the Bargain Mart?"
In these cases, there is only one dash, since the sentence never returns to it's original topic.
The em dash can also be used to indicate speech being cut off, as in dialogue in fiction novels.
"I had this great dream! It was—"
"Don't even talk about it," Jake interrupted harshly.
In the sentence you've presented, the em dash is being used to indicate a change in the structure of the sentence. The second part of the sentence elaborates on the proposed category of "genre fiction". Parentheses or a colon would also have served this purpose, but the writer in this case opted for the em dash.
Besides the Em Dash, there are four other dashes in the English language:
- The Hyphen
- The En Dash
- The 2-Em Dash
- The 3-Em Dash
I won't discuss those here (it would take me pages), but at least now you can understand the em dash, which along with the hyphen is one of the most utilized.