Gelatin - What is it made from?

Here are some descriptions of gelatin.

Gelatin: is made from collagen extracted from the bones and connective tissues of animals such as the domesticated cattle, pigs and horses
(en.wikipedia.org/wiki/Gelatin)

Gelatin: a colorless water-soluble glutinous protein obtained from animal tissues such as bone and skin
(wordnet.princeton.edu/perl/webwn)

Gelatin is a setting agent, derived from animals, available as a powder or a sheet.
(thefoody.com/glossary/glossaryg.html)

That's why for vegetarians it is preferable to use agar-agar, a type of seaweed used in the same way as gelatin.