Equity refers to the ownership interest or residual claim in the assets of a firm after all the liabilities of the firm have been paid. While referring to a firm’s balance sheet, equity refers to the capital contributed by the owners (also known as shareholders) plus any retained earnings or losses.
A firm can issue various types of equity securities to raise capital. Different securities have different ownership claim on the company’s net asset.
Common shares are the most common type of equity securities issued by a company.

Callable and Puttable Common Shares
The common shares may also be callable or puttable. A call option gives the issuer of the common shares the right, but not the obligation to call the shares back at a pre-specified call price which is set at the time the common shares are issued. These shares are also called redeemable common shares. The issuer will generally call the shares when the current market price is higher than the call price. In such a case, the issuer can buy back the shares at the call price and sell them in the market at the market price. Note that it’s a right of the company and the investors holding the shares are obligated to sell if the issuer decides to call back the shares. Calling the shares helps the companies in reducing their dividend payments.
A put option gives the investor the right but not the obligation to sell the common shares back to the issuer at a prespecified price set at the time the shares were issued. Investors will be interested in selling the shares back if the current market price is less than the put price. This helps the investors in lowering their loss potential. Put options are beneficial for issuers as they make their issue more attractive to investors.