In the modern world, the securities markets include the entirety of the international markets and exchanges that facilitate the issuance, buying, and selling of publicly-traded securities. By most estimates, the shares of stock of almost 650,000 companies are traded in these markets, representing a total global capitalization of almost US$90 trillion. Approximately US$10 trillion of that amount is in shares that are traded on the New York Stock Exchange.
In the first quarter of 2020, more than US$32 trillion in share value changes hands on international markets. Between 2 and 6 billion shares are traded every day on the New York Stock Exchange alone. This level of activity is driven by buyers and sellers and is facilitated by market makers and brokerage houses that generally guarantee liquidity and manage trades for both individual and institutional investors. Give the size and value of trading on international markets, most market makers are part of brokerage houses that buy and sell shares for their own account. Many brokerage houses have developed mechanisms for borrowing and lending shares to fulfill their market maker function if their own shareholdings are inadequate to match buy or sell orders.
Markets are further served by custodians and depository institutions that provide control and administrative services to monitor and clear daily trading. The aggregate benefit of the interaction of all of these market participants is that all investors in the markets are able to get the same information about issuers at the same time, thus creating a market for issuer shares with perfect competition and balance.   
Before a company's shares are traded on an exchange, the company must apply to register those shares with the regulatory agency that has jurisdictional oversight over them. In the United States, the Securities and Exchange Commission has the authority to review and approve or deny applications for share registration. Regulatory agencies in other countries have analogous regulatory power according to laws that define the information that a share issuer must disclose for the application to be approved. Companies with lower capitalizations may be able to apply for share registration with a streamlined application process, but a majority of issuers process their registration applications through a more rigorous initial public offering (IPO) structure.
Once an issuer’s shares are registered, the issuer can apply to have those shares traded on an exchange that maintains a physical location, such as the New York Stock Exchange, or in a virtual over-the-counter (OTC) market. Trading in the physical markets occurs only during established trading hours, whereas the virtual markets enable trading around the clock via networks of telephones and computers. The exchanges also require an issuer’s shares to meet certain capitalization and trading standards and an issuer’s shares are subject to being delisted if they fall below those standards.
Shares that are traded on the OTC markets are generally classified into one of three tiers: OTCQX, OTCQB, and Pink Sheet. OTCQX issuers must certain stringent financial standards. OTCQB issuers are typically entrepreneurial companies that are still in their development stage. Issuers in the OTCQB market must submit annual reports and management certifications to maintain their status on that tier. Pink sheet companies are in the lowest virtual market tier and are generally traded with less reporting or involvement by the issuer.
For more than 100 years, the markets for buying and selling securities have democratized access to information and created the conditions that have fostered unprecedented capital creation and growth. Investors benefit from universal price discovery and efficient buying and selling transactions, all within a highly-regulated framework that ensures complete disclosure of information and equitable brokerage services.