Tell them in thirty seconds. This is a great way to hold their attention. If you talk too long while trying to explain what it is (at least before they start asking questions), the people will get confused or lose interest. Also use little words. Do not use jargon, as much as possible. The ordinary people will not understand the technical terms, and it will just make your explanation messy. Hope this helps!
Social Philosophy is the study of questions about social behavior of humans and addresses a wide range of subjects like effects of science on culture. You can also explain to ordinary people that the social philosophy deals with the concept and principles regarding the society. Visit http://en.wikipedia.org/wiki/Social_philosophy for more details.