By TheStreet Staff What Is a Mortgage-Backed Security? Many homeowners think of their mortgage in personal terms. But when grouped together, mortgages represent an investment class that investors—particularly investment banks—trade for profits. Packages of mortgage-backed securities, or MBS, were the catalysts behind the Financial Crisis of 2007–2008, when the mortgage-backed security market imploded from subprime mortgage defaults, sending financial markets into crisis mode. In the years following, the U.S. government added regulatory measures to ensure these investments were less risky, whic…