Gelatine is an unacceptable product to vegetarians as it is a by-product of the slaughterhouse industry, being made of protein derived from animal bones, cartilage, tendons and other tissues such as pig skin. Isinglass, used in fining some alcoholic drinks, is a type of gelatine from the air bladders of certain kinds of fish. Aspic is also unsuitable, as it is made from clarified meat, fish or vegetable stocks and gelatine.
However, there are various alternatives available, which do not contain any animal products whatsoever. These include agar agar, carrageen and a proprietary product called Gelozone.
More info at:
http://www.vegsoc.org/info/gelling.html