Variationist Sociolinguistics differs from Interactional Sociolinguistics as it focuses on social variation in dialects and examines how this variation is highly structured. This structured variation tells us that this is part of human language capacity, a built in system of language in a brain.
 How did variationist sociolinguistics develop?
As you have probably noticed, these two names have popped up a lot in Sociolinguistics. This is because their research has been very influential and have both contributed different ideas to Variationist Sociolinguistics.  
The Sociolinguistic Interview
Labov tried to come up with ways of interviewing people without the issue of the observers paradox.
He collected different styles of speech to overcome this by using techniques where people wouldn’t pay attention to their speech. This is known as unconscious speech.
He also collected conscious speech so he could tell when people were paying attention to their speech.
If this is too confusing for you and you would like a general and basic overview of sociolinguistics then visit What does Sociolinguistics study?
Human language capacity: The idea that people are born with a built in capacity for language, and that people already know language but just need to learn the rules of language, and that people recognise whats right and wrong on how to use language.
Social practice: When speakers engage in an activity together in a group
The sociolinguistic interview: Collecting data from people in an interview as naturally as possible
Observers paradox: When participants alter their language because they are aware they are being observed
Un self-conscious speech: When people don’t pay attention to their speech through telling a narrative.
Very self-conscious speech: When people pay attention to their speech, such as when saying minimal pairs e.g. bat, pat.
 Eckert, P. (2000) Linguistic Variation as Social Practice: The Linguistic Construction of Identity in Belten High. Malden, MA: Blackwell.
 Labov, W. (2008) Quantitative reasoning in linguistics. http://www.ling.upenn.edu/~wlabov/Papers/QRL.pdf