Stock Market
A stock market, to say the very least, is a place for buying and selling stocks. It also acts as an indicator of the economic cycle. When the economy is doing well, the prices of stocks tend to increase in the market, generally. However, when the economy is down, you can also expect the prices of stocks to decline, this can be true even for a very good stock. It is also worth noting that the prices of stocks primarily depend on the performance of the business. When a business is doing well, the price of its stocks will also tend to increase; the opposite would happen if the business is not earning well. Of course, there are odd cases where speculators buying into a stock that is not doing well at all, which will result in a high stock price even for a crappy company, but that is another story for another time. Now, the question remains: Why do businesses offer stocks? The reason is that it helps businesses to raise funds in order to finance their projects, whether for the expansion of the business, or for their day-to-day activities, or just for the sake of it to keep their stakeholders happy. Money, after all, is the lifeblood of every business.
Stock
A stock, also referred to as a share, signifies ownership of a company. When you buy stocks from a company, you get to exercise rights of ownership of the company, such  as  a claim  on  the company’s  earnings  and assets,  as  well as voting rights. Take note that there are many kinds of stocks. But, when you hear investors talk about the stock market, they usually refer to common stocks.
Stock Market Index
When people talk about stocks, you may hear them talk about how the market rises or falls in a day. However, when you take a look at the stock market, you may see that not all of the stocks rise or fall on that day. The way to understand this is to know the stock market index, or indices (plural).