Sociolinguistics is the study of how language is related to society. It also studies how people affect language usage. Sociolinguistics studies how variety in language changes between groups of people because of things such as race, gender, status, age, etc.

Indian and Japanese linguists first studied the social parts of language in the 1930s. Louis Gauchat in Switzerland also studied this in the early 1900s. However, none of them were known in the West until much later. The study of why language changes started during the late 1900's in the wave model. Thomas Callan Hodson was the first person to us the word sociolinguistics. It was in an article he wrote in 1939 called "Sociolinguistics in India". This was printed in a magazine called Man in India. Sociolinguistics in the West first came in the 1960s. It was pushed forward by linguists such as William Labov (in the US) and Basil Bernstein (in the UK). In the 1960s, William Stewart and Heinz Kloss introduced the simple ideas for the concept of pluricentric languages. This explains how standard language varieties are different between countries (e.g. American/British/Canadian/Australian English; Austrian/German/Swiss German; Bosnian/Croatian/Montenegrin/Serbian Serbo-Croatian).

Applications of sociolinguistics 
For example, a sociolinguist might research how a particular variety of language (vernacular) would not be considered correct in a business place. Sociolinguists might also study the grammar, phonetics, and vocabulary of this sociolect. Similarly, dialectologists would study the same things for a regional dialect.

The study of language variation focuses on how language changes with social pressure and when in a specific environment. Code-switching is a word used to explain the language changes in new environments.

William Labov is often thought of as the most important person who studies sociolinguistics. He is especially known for introducing a way to measure language variation and change. This made the study of language into a scientific area of study.

Further reading 

 Cook, Manuela (2019). N-V-T, a framework for the analysis of social dynamics in address pronouns. In: Bouissac, P. (Ed.). The Social Dynamics of Pronominal Systems. Amsterdam: John Benjamins. Chapter 1, pp. 17-34. {Hardbound ISBN| 978-90-272-0316-8}} and {e-Book ISBN| 978-90-272-6254-7}}

Lakoff, Robin T. (2000). The Language War. Berkeley, CA: University of California Press. 
Meyerhoff, Miriam. (2006). Introducing Sociolinguistics. New York: Routledge. 
Milroy, Lesley and Gordon. Matthew. (2003) Sociolinguistics: Method and Interpretation London: Blackwell Publishing. . (More advanced, but has lots of good examples and describes research methodologies to use.)
Paulston, Christina Bratt and G. Richard Tucker, editors. 1997. The early days of sociolinguistics: memories and reflections.‭  (Publications in Sociolinguistics, 2.) Dallas: Summer Institute of Linguistics.
Trudgill, Peter. (2000). Sociolinguistics: An Introduction to Language and Society(4th Ed.). London: Penguin Books.  This book is a very readable, if Anglo-centric, introduction for the non-linguist.
Watts, Richard J. (2003). Politeness. Cambridge: Cambridge University Press.  . A sociolinguistics book specializing in the research in politeness.  It's a little tough at times, but very helpful and informational.

References

Related pages
Linguistics
Sociology

Other websites

http://www.ncsu.edu/linguistics/ncllp/aboutfieldwork.php About sociolinguistic fieldwork
Sociolinguistics: an interview with William Labov  ReVEL, vol. 5, n. 9, 2007.

Branches of linguistics