Diglossia and Polyglossia

"Polyglossia" has been used in situation when  a comunity regularly uses more than 2 languages. They can be different types of high varieties or low.
The term "Diglossia" was introduced by Charles Ferguson in order to describe the situation found in places like Greece. The Arabic speaking world in general Swizeland (Swish, German and England and the Island of Haiti) in all these sociactics there are two varieties distinct enough to call them seperate languages. One is used only on formal and public occasions, the other is used by everybody for everyday communication. E.g. In Arabic speaking world we distinguish between Standart Arabic and colloqual Arabic. In the same way in Swizelamd (Swiz- German). High variety is only learned at school. So diglossia has 3 important features:
I. Two distinct varieties of the same language are used in the community with one regarded as High variety, the other Low.  Each variety is used for distinct feature. No one uses the High variety in everyday conversation. It will sound funny. E.g. Standart German uses "cartofel" for "potato" while Swiz German uses "hadopfel". People admire High variety even if they don't understand it, but they feel more comfortable with Low variety.
Generally diglossia has been described as a stable situation but Low variety may gradually displace the other. E.g. Latin was pushed out from its position in Europe as High variety as the Low varieties expanded and got into more formal domains. E. g. England was diglossic after 1066 when the Normans were in control. French was the language of legal system. English was the language of peasants. By the end of the 14th century English has displaced French, though it should be mentioned we will use the French words.

Register

The term register is widely used in sociolinguistics to refer to varieties according to use.  One and the same person may use different linguistic items to express more or less the same meaning on different occassions. E.g. in writing a letter a person might start. "I am writing to inform you that...", or "I just wanted to let you know that...". In the I case it is more official while the II shows the solidarity with the person you are writing to. Haliday distinguishes 3 general types of dimensions: 

1. field,

2. mode,

3. tenor.

Field is concerned with the purpose and subject matter of communication, in other words, it refers to why and about what the communication takes place. 

Mode refers to the means by which communication takes place, i.e. by speech or writing. 

Tenor depends on the relations between participants. The speaker defines his relationship with the person he is communicating with.

Hymes uses different terms: social distance (solidarity), status or power, formality (the piece of writing is formal or informal language), function (referential and affective function). When we want to give information it is referential and when we express our emotion it is affective. 

Regional and Social Dialects

We distinguish between  regional and social dialects. If the differences are based on the biography, then we identify what are called regional dialects.They are studied by the discipline called dialectology. One of the fundamental findings of sociolinguistics is that class and variety are related. Members of the working class tend to speak less standart language, while middle and upper classes will speak closer to the standart language. So besides biography there exists another important factor-social class. Therefore we may speak of social dialects or sociolects as distinct from regular language dialects.