The end products depend on the type of fermentation. In lactic acid fermentation, used by muscle cells short of oxygen, red blood cells, and bacteria such as those in yogurt, pyruvic acid is converted to lactic acid (lactate). In alcohol fermentation, used by yeast in bread, beer, and wine, pyruvic acid is converted to ethanol and carbon dioxide gas.
Both pathways also regenerate NAD+, which lets glycolysis keep producing a small amount of ATP without oxygen. Bacteria carry out other kinds of fermentation as well, and most of them release a gas; lactic acid fermentation is the usual exception.
Source: OpenStax, Biology 2e, “7.5 Metabolism without Oxygen”
Image: Rising bubbles from yeast fermentation, by Jim Champion, CC BY-SA 2.0, via Wikimedia Commons.