(Jain = follower of the Jinas, the overcomers)
An Indian belief system that was invented by Mahavira at around the same time as the invention of Buddism. It stresses ahimsa (non-violence), which is seen as the most essential religious duty. It is deemed Nastika because it rejects the authority of the Vedas.
1. Here's a good summary of their basic teaching: "The phenomenal individual consists of a soul closely enmeshed in matter, and his salvation is to be found in freeing the soul from matter, so that it may regain its pristine purity and enjoy omniscient self-sufficient bliss for all eternity."
(Sources of Indian Tradition, 46)
2. They are reknowned in India for their skill as businessmen, and are famously honest. In addition, cow fetishism and vegetarianism might have originated with them.
3. It is a much more physically demanding faith than Buddhism, which is probably why the latter was more successful. For example, Buddhists shave their heads, whereas Jains pluck their hair; Buddhists beg for food, whereas Jains aren't even allowed to beg — food must be given to them.
4. Jains believe that everything has a soul and feels pain, including inanimate matter such as rocks. Hence, their formulation of ahimsa, which follows from that empathetic logic.
5. Diwali has special significance in Jainism: It marks the anniversary of Mahavira's achievement of moksha.