A "REIT" is a real estate investment trust. REITs are companies that own -- and typically operate -- income-producing real estate or real estate related assets (i.e. mortgages). REITs are designed to be tax efficient: in order to qualify as a REIT, at least 90% of taxable income must be paid out to shareholders annually. For this reason, REITs typically offer attractive dividend yield, as well as long term capital appreciation as real estate values rise.
While most REITs are publicly traded on stock exchanges, some are privately held. REITs were first created in the 1960s and now represent a multi-trillion dollar asset class. Equity REITs tend to own one specific type of real estate -- single family residential, multi-family residential, office, industrial, retail.
For more information about REITs, visit Nareit, the National Association of Real Estate Investment Trusts.