How do different securities markets and market participants operate?

"Broker markets: National and regional securities exchanges that bring buyers and sellers together through brokers on a centralized trading floor. Dealer markets: Securities markets where buy and sell orders are executed through dealers, or “market makers,” linked by telecommunications networks. Electronic communications networks (ECNs): Private trading networks that allow institutional traders and some individuals to make direct transactions in the fourth market. Exchange traded fund (ETF): A security similar to a mutual fund; holds a broad basket of stocks with a common theme but trades on a stock exchange so that its price changes throughout the day. Institutional investors: Investment professionals who are paid to manage other people’s money."