Culture refers to the collective accumulation of knowledge, beliefs, practices, traditions, art, representation, and experiences that get associated with a defined group of people and transmitted from generation to generation.
Multiculturalism refers to the cultural diversity of communities. As an idea, it promotes the understanding that society is comprised of distinct cultural communities that should be afforded agency, sometimes even self-determination or autonomy, typically within an institutionalized framework - the state, the corporation, the university, the organization, etc.
Polyculturalism refers to the interchange that occurs between cultures. It assumes cultures to be fluid and evolving, not static and fixed. As an idea, it promotes the understanding that society is comprised of cultural communities that continuously intersect and redefine themselves through processes of dynamic engagement.
Unlike multiculturalism, polyculturalism is less adaptable to public policy and organizational rubrics, and more rooted in voluntary, often spontaneous, exchange and discovery. Polyculturalism is more concerned with the personal evolution that creates cultural and societal change, whereas multiculturalism is more concerned with preserving cultural integrity and managing relationships between communities.