Just what can be done with someone's Social Security Number?

All throughout my life, I’ve been warned about my SSN. I need to keep it safe, I need to not tell it to people, I need to memorize it and not carry it anywhere on my body, so that nobody can steal it.

But as I slowly creep into the world of adulthood, I’m finding a lot that I do need to give this magical number to people. I had to give it over the phone yesterday to someone in the IRS. I had give it to someone at school today to order something. I have to keep my Military ID card with me at all times if I want to get into the base (and the Military ID does have SSN on it). Employers need it. Landlords need it.

I know that this number is unique, but I have no idea what it even does. All I know is that I’m supposed to simultaneously protect it from some people, and apply it liberally to other people.

If this number falls into the wrong hands, what exactly can happen?