Norms are expectations set by people in a society that express that societies values, beliefs, and traditions. These norms are what the people who live in a society (typically) adhere to and are guided by. It’s also important to note that not every society or ethnic group has the same set norms.
While this idea is shifting in our society, because norms can change throughout time, some consider homosexuality to be breaking the norm of “conventional” heterosexuality or the male and female relationship.
In Butler’s writing about John/Joan she brings up how abnormal John felt with all the intense gender expectations to be a “normal” girl. All the while John didn’t want to play with dolls and he felt “masculine” which is not the “norm” for a woman. In our society we have a lot of gender norms that dictate how woman and men are supposed to act or look. As an example, there is the norm that women should have long hair and in turn that men should have short hair. Also that only men should play baseball and girls should play softball.
While different people can agree or disagree with set norms in the societies they live in, there is a lot of pressure to conform to norms and when someone fails to do so they could be considered deviant or abnormal.
A Feminist Theory Dictionary 
We have a pretty good idea why people adhere to norms. After all, one faces a great deal of social stigma by deviating from established behavior (and who wants to put up with all that grief?).
The main questions (I think) remain unanswered. Who is it that manufactures norms, and, more importantly, how is it that norms are changed? You address both issues in the definition, but can we go deeper? These questions are intimately linked. Again, there is no wizard. The intentionality involved is weak, at best.
And yet there is likely a logic to how norms rise and fall in our societies. Is it simply historical precedent? “Women have had long hair forever, so that is why they face mild ostracization today if they have short hair.” No, because this doesn’t leave open the possibility that norms can be altered. Are norms just the products of the ebb and flow of history? Or as humans do we shoulder some responsibility, and thus agency, in the creation and duration of norms?