The New York Stock Exchange used to have these people called specialists. Each one was assigned a handful of stocks and was basically the boss of all trading in those stocks. They stood at a physical post on the floor, saw every order coming in, ran the opening auction, and traded with their own money when nobody else would. One of the most privileged positions in finance. And one of the most controversial.
Dealers are merchants. They buy low, sell high, pocket the difference. If you ever bought a used phone from a resale shop, you understand the concept. The shop bought it for less, sells it to you for more. Financial market dealers do the same thing with stocks, bonds, and currencies.
The specialist system is one of those things that sounds simple on paper but gets really complicated in practice. And controversial. Very controversial.
Everyone talks about liquidity. Traders talk about it. Regulators talk about it. Financial journalists definitely talk about it. But Harris makes a sharp observation right at the start of Chapter 19: rarely does anyone define what they actually mean. People use the same word to describe different things, and then they wonder why they cannot agree on anything.
Book: Trading and Exchanges: Market Microstructure for Practitioners Author: Larry Harris Publisher: Oxford University Press, 2003 ISBN: 0-19-514470-8