Note that fermentation itself yields no ATP molecules; it serves to recycle NADH back to NAD^+, which is reused in glycolysis.
Fermentation review: 
In anaerobic conditions (absence of oxygen), some cells undergo fermentation instead of aerobic respiration.
After glycolysis, pyruvate is broken down by fermentation in the cytosol via two catabolic pathways:
Note that fermentation itself yields no ATP molecules; it serves to recycle NADH back to NAD^+, which is reused in glycolysis.
Fermentation review: