There are two primary theoretical marketplaces where securities are traded. The term theoretical is used here because the market need not be a physical place where buyers and sellers meet to trade securities. This is akin to references to different markets for selling both new and used automobiles. The primary market for car sales refers to sales of cars between the manufacturer (or a dealership) and the buyer. A secondary market refers to used car sales and may be between any owner and seller. The point is that primary markets refer to new sales and secondary markets refer to used sales.
A stock exchange is any legally recognized market where securities can be bought and sold. Corporations that issue stock are traded in these markets and typically wish to be listed as an officially traded company so that the shares of stock can be purchased by investors in a liquid market. In addition, stock exchanges are regulated such that both companies and investors must follow trading rules making the transition of ownership safer and less risky. The liquidity of the market refers to the ease with which ownership of a company in the form of stocks can be bought and sold without delay or complications afforded by selling the stock on a one-to-one basis between each company and investor.