An ETF designed to replicate the performance of the Standard & Poor's 500 Index. Because of its acronym, the SPDR instrument is referred to as a "spider."

Mortgage-backed securities (MBS) are debt obligations that represent claims to the cash flows from pools of mortgage loans, most commonly on residential property. Mortgage loans are purchased from banks, mortgage companies, and other originators and then assembled into pools by a governmental, quasi-governmental, or private entity. The entity then issues securities that represent claims on the principal and interest payments made by borrowers on the loans in the pool, a process known as securitization.

"Net asset value," or "NAV," of an investment company is the company's total assets minus its total liabilities. For example, if an investment company has securities and other assets worth $100 million and has liabilities of $10 million, the investment company's NAV will be $90 million. Because an investment company's assets and liabilities change daily, NAV will also change daily. NAV might be $90 million one day, $100 million the next, and $80 million the day after.