Diglossia (雙層語言)

When I was surfing "Deutsch-lernen.com", there is a page telling the relation between Switzertütsch and standard German is a diglossia. Looked up the dictionary, diglossia this word means to me that a second language spoken in a country. But what is the difference between diglossia and bilingual? After digging out, diglossia usually is divided into high-variety and low-variety. High-variety is spoken in formal and low-variety is what we speak in daily communication. Bilingual means total different language system.