“It begins with the simple act of noticing a variation – that there are two alternative ways of saying the same thing” 
Labov’s quote here is pretty self-explanatory of what sociolinguistic variation is: it is simply different words, sounds and language people use to explain the same thing.
These variants (different ways/ ‘realisations’ of saying the same thing) can
be lexical (words) or phonetic (sounds of the letters in the word).
What sociolinguistics are interested in is seeing which variants are used, and deemed more appropriate for use, in certain social contexts.
Which of the following would you say to your TEACHER?
Which of the following would you say to your MUM?
Which of the following would you say to your FRIEND?
Which of the following would you say to A STRANGER?
 Labov, W., (2008). Quantitative Reasoning in Linguistics. Available from: <http://www.ling.upenn.edu/~wlabov/Papers?QRL.pdf> [Accessed 08.05.2012]