How does Buddhism define abortion?

Abortion is a much-debated topic, and religion frequently plays a role in any such discussion. How does abortion fit into Buddhist faith? Is it considered negative karma by definition, or perhaps negative karma, depending upon the situation (e.g. in a family that cannot afford to properly support and raise a new child, or in the case of an abortion to save the life of the mother)? Are there definitions of when human life begins tied with Buddhism?