A BROKER

A "broker" is merely a representative of a buyer or seller. Brokers manage individual's portfolios, provide advice to those individuals about what to buy and sell, and then place "buy" and/or "sell" orders with the "stock exchange" on behalf of their clients. Brokers generally receive a fixed fee or commission based fee for these services.

A company "sells shares into the market" in order to raise money.
In broad terms every company get's it's cash from three main sources:
(a) Equity (selling shares into the market);
(b) Debt (getting loans from banks and other sources); and
(c) Operations (making cash via it's operations).

If you were to be philosophical, when a company "sells shares into the market", it is really "loaning money from shareholders" (although it is not technically a "loan"). So when a company "sells shares into the market", they would use the term "raising equity capital in the market". The big difference with "Shares", is that "shareholders" are the "owners" of the company, and as such they (generally) face the highest risks in relation to rate of return (dividends) and liquidation scenarios, amongst other things.

"Stock exchanges" have a critical role in the economy on both a domestic and global scale. Without "stock exchanges", there would not be a liquid market in which to trade shares in companies, which would dramatically change the nature of the global economy. It is also worth noting that when a company "lists" on a stock exchange, they are subjected to strict rules that require the company to disclose a lot of information about their financial results (amongst other things) to the public. It is this information that fundamentally drives a lot of enterprise and economic analysis.