Difference between "spine" and "backbone"

Backbone and spine are both common names for the vertebral column, the series of bones from one's head to pelvis. In my personal opinion, backbone comes across as more of an informal name than spine.

It is worth noting that both of these words, however, have other meanings as well:

• Spine can also be used to refer to a needle-like protrusion on plants and animals (among other things) such as a thorn. It can also reference the bound edge of a book.
• Backbone can also be used to refer to nearly any fundamental support structure. For example, an internet backbone, which is the top level of inter-connectivity between core pieces of the internet.

The term "backbone" is pretty much restricted to a single idiom: to "show some backbone," which means to be brave or resolute.

In this phrase, you can substitute the word "spine," and the meaning is unchanged. In almost any other conceivable instance, you'll want the word "spine," as in, "she injured her spine in the accident."

Edit: The above only refers to mammalian physiology. Figurative or metaphorical uses of "backbone," such as "ATT is the backbone of the Internet" or such is a different matter.

As reported by the NOAD, the meaning of backbone is:

• the series of vertebrae extending from the skull to the pelvis
• (figurative) the chief support of a system or organization; the mainstay
• (figurative) strength of character; firmness
• the spine of a book

For those meanings, there is a similar meaning for spine.

In zoology and botany, spine is also any hard pointed defensive projection or structure, while in geology it means also "a tall mass of viscous lava extruded from a volcano."
In biochemistry, backbone means also "the main chain of a polymeric molecule."