Diglossia refers to a sociolinguistic phenomenom where the standard and vernacular language of a nation suffers changes in usage per region, or by the unique use of a particular group. As a result, one country may speak one language, but this language will sound differently from one region to another.
Another meaning for diglossia is the different use of the same language by specific situation. The "H", or "high" language variety refers to the use of the language in a formal setting, while the "L" or low variety refers to using the language everyday. In either manner, diglossia is the dual use of one same language.
The colloquial transformation of a language is a cultural expectation. Cultural groups that share similarities will undoubtedly transform their communication and fit it to their unique traits. An example of diglossia in the United States is the National Standard English, versus the transformations that can be perceived in the use of Southern and Midwestern English.
Again, diglossia is not limited to the English language. It is seen in every language and cultural group and it is a phenomenon which will continue to occur as new groups form.