Focus groups simply discussion groups composed of members of the population you want to study. They are an exploratory research method used to generate hypotheses to test in further quantitative research.
Focus groups can also be quantitative used as a survey to test statistical hypotheses. Using focus groups in this way is difficult, as interpreting the results requires analytical sophistication.
Drawbacks can be: representative samples, time requires, attendance, influence of comments from paid focus groupies and conformity, moderator skill, using quantitative questions
Use focus groups when you need to determine what questions to use in subsequent formal research. Allow participants decide what to talk about, and let them discuss it without any intervention excellent to maintain order or to clarify understanding. Analyze the resulting transcripts thematically rather than quantitatively.