Pluralism or tolerance is the idea that we maintain whatever social identities we came into the country with without abandoning them and fully assimilating. For example, some people came here who were Protestants, some who were Catholicis, other Buddhists, Hindu, Muslim. Should we expect these people to abandon their religions? And what would they abandon it for? There is no national religion in America. To this day many Italian and Polish Americans still retain a sense of their ethnic identity despite being more or less fully assimilated into American culture.That you have the freedom to practice your religion. That is pluralism and tolerance.
Multiculturalism This is a perspective that we must not only tolerate the existence of other social identities, but that we should also know about them and appreciate how they are different from what ever our own identities may be. This is the whole premise behind events like multicultural fairs where you get the opportunity to sample other foods, like indian curries or middle eastern kabobs. This is also the premise behind multicultural parades where different groups show off elements of their culture (dress, dance, etc.). I think these are good events because they do increase our knowledge of other cultures which helps to maintain pluralism and intergroup harmony by making other groups seem less mysterious and strange.