Diglossia can be presented as two versions of the same language used for different social contexts (Arabic is the language that is prototypical for this). Bilingualism means different things to different people. Generally it means an individual who commands and uses two languages. In language proficiency this means two languages at the same level of mastery. At sociolinguistics level this just means two languages. Often diglossia is used to talk about the use within society. The problem comes when some use the term bilingualism within the society. It is rare to find the term diglossia outside of sociolinguistics. Bilingualism is used by psychology, sociology, education, and policy makers--thus the more generic and less tight defintion or functional use of the term.
