What is taught, how it is taught. All the experiences of the student for which the school is responsible. Includes subject matter, materials, teaching methods, interactions, and social experiences. Each state has own plan. Centralized vs. Decentralized US is somewhat unique in that individual states designate curriculum.
Most industrialized nations have a centralized curriculum, that is a national curriculum.
On a national level, the US can be said to have a decentralized curriculum, no national plan. However, curriculum is centralized, state by state. The official learning plan of the schools. The formal curriculum is probably on file somewhere and is usually fairly explicit. However, teachers often have a great deal of latitude in choosing how they will teach a particular subject.