No language is a uniform system in which everyone talks just like everyone else. The field of sociolinguistics investigates relationships between social and linguistic variation (Romaine 2000; Trudgill 2000).
How do different speakers use a given language? How do linguistic features correlate with social diversity and stratification, including class, ethnic, and gender differences (Tannen 1990; Tannen, ed. 1993)? How is language used to express, reinforce, or resist power (Geis 1987; Lakoff 2000)?
Sociolinguists focus on features that vary systematically with social position and situation. To study variation, sociolinguists must observe, define, and measure variable use of language in real-world situations. To show that linguistic features correlate with social, economic, and political differences, the social attributes of speakers also must be measured and related to speech (Fasold 1990; Labov 1972a).