A corporate action is an event initiated by a public company that affects the securities (equity or debt) issued by the company. Some corporate actions such as a dividend (for equity securities) or coupon payment (for debt securities (bonds)) may have a direct financial impact on the shareholders or bondholders; another example is a call (early redemption) of a debt security. Other corporate actions such as stock split may have an indirect impact, as the increased liquidity of shares may cause the price of the stock to rise. Some corporate actions such as name change have no direct financial impact on the shareholders. Corporate actions are typically agreed upon by a company's board of directors and authorized by the shareholders. Some examples are stock splits, dividends, mergers and acquisitions, rights issues and spin offs.
The primary reasons for companies to use corporate actions are:
Return profits to shareholders: Cash dividends are a classic example where a public company declares a dividend to be paid on each outstanding share. Bonus is another case where the shareholder is rewarded. In a stricter sense the Bonus issue should not impact the share price but in reality, in rare cases, it does and results in an overall increase in value.
Influence the share price: If the price of a stock is too high or too low, the liquidity of the stock suffers. Stocks priced too high will not be affordable to all investors and stocks priced too low may be de-listed. Corporate actions such as stock splits or reverse stock splits increase or decrease the number of outstanding shares to decrease or increase the stock price respectively. Buybacks are another example of influencing the stock price where a corporation buys back shares from the market in an attempt to reduce the number of outstanding shares thereby increasing the price.
Corporate Restructuring: Corporations re-structure in order to increase their profitability. Mergers are an example of a corporate action where two companies that are competitive or complementary come together to increase profitability. Spinoffs are an example of a corporate action where a company breaks itself up in order to focus on its core competencies.
Corporate actions are classified as voluntary, mandatory and mandatory with choice corporate actions.
Mandatory Corporate Action: A mandatory corporate action is an event initiated by the corporation by the board of directors that affects all shareholders. Participation of shareholders is mandatory for these corporate actions. An example of a mandatory corporate action is cash dividend. All holders are entitled to receive the dividend payments, and a shareholder does not need to do anything to get the dividend. Other examples of mandatory corporate actions include stock splits, mergers, pre-refunding, return of capital, bonus issue, asset ID change, pari-passu and spinoffs. Strictly speaking the word mandatory is not appropriate because the share holder person doesn't do anything. In all the cases cited above the shareholder is just a passive beneficiary of these actions. There is nothing the Share holder has to do or does in a Mandatory Corporate Action.
Voluntary Corporate Action: An voluntary corporate action is an action where the shareholders elect to participate in the action. A response is required by the corporation to process the action. An example of a voluntary corporate action is a tender offer. A corporation may request share holders to tender their shares at a pre-determined price. The shareholder may or may not participate in the tender offer. Shareholders send their responses to the corporation's agents, and the corporation will send the proceeds of the action to the shareholders who elect to participate.
Sometimes a voluntary corporate action may give the option of how to get the proceeds of the action. For example in case of a cash or stock dividend option, the shareholder can elect to take the proceeds of the dividend either as cash or additional shares of the corporation. (these are commonly known as Mandatory Events with Options, as a dividend is mandatory but a shareholder has the option to elect for the cash or to re-invest their cash dividend into the shares) Other types of Voluntary actions include rights issue, making buyback offers to the share holders while delisting the company from the stock exchange etc.
Mandatory with Choice Corporate Action: This corporate action is a mandatory corporate action where share holders are given a chance to choose among several options. An example is cash or stock dividend option with one of the options as default. Share holders may or may not submit their elections. In case a share holder does not submit the election, the default option will be applied.
Corporate Actions Information 
When a company announces a corporate action, registered shareholders are told of the event by the company's registrar. Financial data vendors collect such information and disseminate it either via their own services to institutional investors, financial data processors or via online portals in the case of individual investors.
Corporate Actions Glossary: 
List of voluntary corporate actions: 
ISO15022 MT564 message format for corporate actions data messages:
SIX Telekurs Corporate Actions data offering: 
Mergent Corporate Actions data offering in REST API form: 
Assessing the Risk in the Corporate Actions Process: Industry Insight 
Everything about Corporate Actions: