Ethnomethodology is a method for understanding the social orders people use to make sense of the world through analysing their accounts and descriptions of their day-to-day experiences - wikipedia 
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Ethnomethodology is a descriptive discipline and does not engage in the explanation or evaluation of the particular social order undertaken as a topic of study.
In essence ethnomethodology attempts to create classifications of the social actions of individuals within groups through drawing on the experience of the groups directly, without imposing on the setting the opinions of the researcher with regards to social order, as is the case with sociological studies.
History
The term itself was originally coined by Harold Garfinkel in 1954, but the discipline itself has changed from Garfinkel's original programme to the point where it is no longer recognisable.
It is an alternative to the American sociological approach to data analysis born in the 1960s with its theoretical and epistemological importance being that it is a radical breach from traditional sociological modes of thinking.