A REIT is a fund that pools together the capital of multiple investors to invest in real estate. A given property may cost too much or prevent investors from properly diversifying if they were to purchase the property themselves. In these cases, they can create a REIT and pool the money together to buy the building and hold the building's deed within the trust. Then, when the trust collects rental income, they'd be able to distribute the money back to the initial capital providers.
The key factor of a REIT is the liquidity it offers. When you invest in a REIT, you're investing in shares of a fund or holding company which is the real owner of the property. The fund or holding company typically needs to create disclosures and documentation outlining what the REIT intends to invest in (ie. a Prospectus).
The Nophin blog details information about the differences between Active Real Estate Investing and Passive Real Estate Investing.