Sociology is the study of the behavior of people as members of society. It studies the relationship that exists between human beings and their societies. People influence their societies and their societies influence them. Sociology studies this interaction. Sociologists study things like the way that being members of a specific group (women, for example) influences peoples lives. It studies how society helps to cause delinquency and how that delinquency impacts society.
The other social sciences study other aspects of human behavior. For example, psychology studies the inner workings of the minds of human beings. It looks at how we react, in our own minds, to various outside stimuli. Economics looks at human behaviors, but only in the context of the way that they choose to use their scarce resources. Political science looks at the structures that people form to govern themselves and the ways in which they interact with those structures.
Sociology is different from the other social sciences then, in that it is the only one that is specially concerned with the interaction between human beings and their societies.