The stock market was first established in LY 902, concurrently with the introduction of banks. Both innovations comprised an integral part of The Plan, which was designed to bring the world closer together, ultimately leading to the establishment of the Second Order.
Ever since money came into usage in 107, it became an invaluable tool for the development of society as the world's population increased, and jobs became more specialized. Over time, some people began earning more money than others, and eventually, companies would be formed. These companies would typically be owned by an individual, or a clan, though they might be governed by unrelated partners who had pooled their resources to found the company. Thereafter, such companies might continue under the leadership of those who founded them or their descendants, though it was not uncommon for companies to come under the leadership of a "board of directors," who might obtain their position by various means. Aside from inheriting a seat on the board, they might buy their way into the company, or work their way up the ranks from a lower position within the company. Membership on a company's board might also change if a number of smaller companies merged. In any case, it was rare for any individual to attain even partial ownership or voting rights in a company if he or she did not have considerable wealth to begin with.
This state of affairs contributed to the class divide, which by 902 was of increasing concern to many of the world's people. Such concerns were largely fostered by The Order, in order to make the public receptive to various ideas they suggested at that time. One of these ideas was the stock market, which would allow anyone to purchase shares of stock (or ownership) in companies. The value of these shares could rise or fall, depending on the profitability of the company. The buying and selling of stock is collectively known as "trading." While the average person couldn't typically afford very many shares, it was nevertheless an opportunity for investment, which is something that had previously been impossible for all but the relatively wealthy. (It should be noted that, as was the case in the earliest days of money itself, many people who made only small investments in the market saw trading stock as a kind of game. And indeed, it bears a strong similarity to gambling, except that random chance tends to play a smaller role in the changing value of stocks.)
The establishment of the stock market not only allowed people of lesser means to invest in existing companies, it also led to an increased number of people pooling limited resources to start new companies. Of course, unlike similar origins of larger companies in the past, which were founded by collectives of a few business people who each had a fairly large amount of money, newer companies could be started by many people who each had a fairly small amount of money. (One of the earliest examples of this was the funding of the Protestant Movement's navy, ironically.)
In spite of the benefits the stock market provided to the lower class (or at least the lower middle class), it did lead to certain problems in the business world. For one thing, there was no rule that said existing companies had to "go public," but many customers of various companies saw it as suspicious if a company refused to offer shares for sale. It seemed to them that the owners of such companies were likely just trying to preserve the gap between social classes, in keeping with the old adage that "the rich get richer and the poor get poorer." This belief led to boycotts of several companies, in favor of newer, smaller companies which did offer stock for purchase by the general public. To mollify the public's perception of them, many larger companies finally relented, though this often came down to a very close vote of companies' boards of directors. Not only that, but public trading of stocks allowed members of one company to acquire voting rights in rival companies, which led to some hostile takeovers of certain companies. Even when stocks were purchased by people who were already on a company's board, it could lead to a shift in the balance of power among directors, or even to a board taking control of a company from its founders (or their descendants). Such underhanded dealing led to the Business Management Regulation Act, which was designed to protect the interests of the rightful heads of companies.