Please define FAR and FSI mostly used in real estate.?

FAR (FLOOR AREA RATIO) and FSI (Floor Space/Surface Index) terms mostly used by builders or concerns. they normaly used terms like I have purchased FSI in particular township or some one wants to sale 50% FAR in his project and rest will be developed by himself. So what is the term FAR and FSI please define it propertly and throughly.