I'm not a lawyer, but I know enough about law to know that NDA's are intended to protect intellectual property, like the design of a new car or the plot of an upcoming movie. They're part of civil law, so breaking an NDA won't land you in prison. It might get you fired or leave you open to lawsuits asking for ridiculous amounts in "damages," though.
NDA's cannot be used to hide illegal behavior, such as sexual harassment, tax dodging, or creating a hostile work environment. (Well, they CAN, but a company will be in a lot of trouble if they get caught.)
NDA's are sometimes written in a way that leaves employees vulnerable to being screwed—like the ones that lay claim to every idea someone has while working at a company, and then forbid that person from working in their field for X years after they quit. That is, long enough for their knowledge to become irrelevant. Unfortunately, these kinds of NDA's have held up in court. The attitude is, "Well, if you didn't like it, you shouldn't have signed it," which makes sense until you realize that entire industries all demand that employees sign similar NDA's before hire. (Tech, automotive, and entertainment companies seem to be the worst.) With our current "pro-business" Supreme Court, I wouldn't expect change any time soon.
I don't know if that helps at all, but that's about what I know.