You'll see a number of groups identified at various points. Lists of them usually start with something like "race, color, religion..." These are known as 'suspect classifications' and discrimination is often not possible except where a suspect class is involved.
Suspect classes themselves have been defined by the Supreme Court via 14th amendment rulings, and include specific criteria that determine what level of scrutiny is applied to the law. If a victim is not a member of a suspect class, then it will be very difficult to prove discrimination. (Though it can happen, for example Gay Marriage in MA)
Companies have a great deal of freedom to 'discriminate'(defined in global scope) in hiring practices, though their ability to 'discriminate'(defined in legal scope) is limited for good reason.
(I am not a lawyer and this is not legal advice. I also don't think puzzle tests should count as discrimination.)
Most people think discrimination is bad. I agree with them. But just because something is bad does not mean that it's correct to make a law against it. The people who discriminate disagree about whether discrimination is a good or bad idea.
Disagreement should not be dealt with by force, it should be dealt with by persuasion, (and with competition). The only exception is when force would happen no matter what we do (ie, they want to assault or defraud someone, then force takes place whether we stop them or not).
When people want to peacefully do things we think are bad, like discriminate, or watch Desperate Housewives, or buy homeopathic medicine, the best thing to do is leave them alone. Do we really want the Government monitoring and judging our peaceful economic choices? The only thing at stake with discrimination is it prevents cooperation between certain people. But so what? I do not have a right to have others work with me; such things should be purely voluntary. And there are still plenty of better, non-discriminating people to work with.
