To say that "norms" are only or primarily about rules is not accurate. An electron moving in an electric field follows strict rules, but we do not say that these rules are "normative." A Euclidean line follows the axioms strictly, and the axioms are rules, but again we do not say these rules are normative.
Even if we restrict the subject to human behavior, still, there are many rules we would not call norms. Suppose a serial killer, deranged as he is, devises rules for himself about how he will conduct his crimes. For instance, he makes a rule that he will always cover the victim's face before killing him. We would not call this rule a norm. We would also not say the serial killer "ought to" behave that way, a closely related concept.
"You should do this," "you should do that." That's what norms are about. "Don't drive on the sidewalk. Don't shoot the dog. Pay your taxes." That kind of thing. Those are norms.
(Social) norms are specifically rules that groups of people typically follow, that they will censure others for not following. Norms are "the normal way things are done" in a given group.
The word "normative" goes a bit beyond the word "norm," in that the word "normative" often connotes more of a sense that you should or ought to do something. "Norm" can be merely descriptive of a group's customs, without judgment of whether those customs are good or bad. "Normative" is typically used in a prescriptive way. It may be a norm in a certain society that the punishment for stealing is to cut off the thief's hands. But "from a normative perspective," a person could claim that this punishment is wrong; he is prescribing how the group should behave, as distinct from how it does customarily behave.
So, what does "should" mean? To rephrase, when a person says, "You should do X," what state of mind does the speaker have, that motivated him to make the statement?
- We can reasonably guess that the speaker is trying to get you to do X. He wants you to do X.
- The speaker thinks, or is trying to convince you, that you doing X is not just a matter of what he wants. He may be trying to convince you that if people in your situation do X, the group as a whole is better off. Or he may be trying to convince you that doing X is customary in your situation, and your peers and superiors would look at you funny if they found out you didn't do X.