Gelatin is used as a gelling agent in all kinds of things and is generally derived from the collagen in animal bones and skin (particularly cows and pigs). Broken down, it's just a mixture of peptides and proteins. But it's still derived from animals, which means there is a risk, however slight, that it could provoke immune system responses in humans or carry infectious diseases. Moreover, animal gelatin can be inconsistent from batch to batch, giving headaches to quality control managers at production plants. And it's not vegetarian.