Cultural Diffusion: The process by which cultural elements spread from one society or group to another. For example, the adoption of sushi in Western countries is an example of cultural diffusion.
Acculturation: The process of adopting certain aspects or traits from another culture while still maintaining some aspects of one's own culture. An analogy for acculturation would be blending different colors together to create new shades.
Assimilation: The complete integration and absorption of an individual or group into another culture. It is like melting different kinds of chocolate together until they become indistinguishable.