Sexism is the view that one sex is superior to the other. It is most often used to mean misogyny, or the belief that men are superior to women. While the view that women are superior to men is also sexism, only in recent years has an awareness of this "reverse sexism" begun developing in public discourse.
More precisely, sexism is a species of collectivism, which holds that individuals can be understood (and often judged) based on the characteristics of the group to which they belong, in this case, their sex group (male or female).
Discrimination based on sex is illegal in many countries. Most countries have laws that give special rights or privileges to one sex (feminists claim: mostly to men; masculists claim: mostly to women). Some countries ban sex discrimination but still have preferential laws in their law books.