This document defines and describes the market for mortgage-backed securities. It discusses the types of mortgage securities issued in the US market, including non-derivative products like mortgage-backed bonds and pass-through securities, as well as derivative products like collateralized mortgage obligations. It also outlines the segmentation of the mortgage market, describing how mortgages originate from lenders and are packaged and sold in the primary market to government agencies, before being traded and resold to investors in the secondary market.