A focus group is one of several techniques used to measure public opinion when devising strategies for propaganda campaigns. Focus groups are often used in tandem with opinion polls, in which PR specialists survey a representative sample of their "target population" and ask each member of the sample an identical set of questions. When a large enough sample is queried, the pollster can claim to have "scientifically measured" the current state of opinion with regard to those questions.
Polls give a more or less definitive, but also fairly shallow, indicator of what people think with regard to questions such as, "Do you support the president?" or "Do you prefer Coke over Pepsi?" Typically the questions are asked in such a way that the answers can be quantitatively tabulated, e.g., 57% said yes, 43% no. By their very nature, opinion polls don't deal well with open-ended, in-depth or complex questions.
A "focus group," by contrast with opinion polling, is a technique that attempts to get an in-depth sense of how people think. Rather than interviewing a large sample using standardized questions, the researcher selects a small group of individuals and leads them through open-ended interrogations aimed at eliciting not only which answer they choose but why they choose it. If you prefer Pepsi, for example, the focus group might ask if your preference is based on the color of the cans or their feelings about Britney Spears. By understanding underlying constellations of attitudes and perceptions, marketers hope to be able to better "focus" their message.