GM Id's

I've read different posts on GM Id's, and don't really know what they are or how they work.
Can someone enlighten me on what they are and how to obtain one?
What they are good for?
Who can use them?
How long do they last?

You can PM me if you don't want to spill all the beans on the open forum.

[f5fstop]

Do a search....It has all been discussed many times