Multiculturalism is a way of life here in America....one of the major building blocks of our society.
There is more to multiculturalism than just having a country of immigrants and their descendants, enriched by the various traditions they brought with them. Multiculturalism, as a philosophy or a government policy, basically says that all of those imported cultures are equal to the dominant local culture. In effect, there is no "official" local culture. That's what we've got in Canada.
In contrast, there is the "Great American Melting Pot" which is not multiculturalism, although people do confuse the two. In a melting pot, everything runs together and becomes part of a homogenous whole. Multiculturalism is more analogous to tossing solid pieces into the same bucket, where they might mix together if conditions permit, but you aren't going to heat it or stir it.