Salary Sacrifice

Received a brochure in the mail about salary sacrifice, but its not really clear exactly as to what it is or how it works.
Does anyone have an understanding of how it works and whether it is a good thing or not? I only have a very basic understanding of the concept. What are the benefits? What are the negatives?
Does anyone have an understanding of how it works and whether it is a good thing or not? I only have a very basic understanding of the concept. What are the benefits? What are the negatives?