What is Sociology?

Broadly speaking, Sociology is a study of the society. This includes individual people, groups of people, interactions, cultures and so on. Sociology looks at things like families, schools and other institutions like criminal institutions. The aim of Sociology is to understand how society works and to break down the mundane practices to see the underlying reason that society is the way it is (especially if we're "doing" Sociology from an Interactionalist point of view).
There are different theories within Sociology, with different groups of Sociologists and reasonings; these are groups like Functionalists (Durkheim, Giddens etc.), Interactionalists (Goffman, Garfinkel etc.) and Marxists (Marx, Engels etc.). There are "founding fathers" of Sociology who founded these groups too.
In order to study Sociology, we must have a critical eye of the world around us in order to understand what is going on. We need to understand that society is a social construction (made and maintained by us) and within society there are a lot more social constructions.