Securities are investment instruments. As a general rule of thumb, if the investment can be easily bought or sold, it's a security. Stocks, bonds, and mutual fund shares are all examples of securities. Real estate property may be considered a security, but that's an example of a less clear issue. Property owners don't always see their property as an investment vehicle they trade. If real estate property is divided into shares of a REIT, on the other hand, then those shares are real estate securities.