Microcontinuity 2.0 by Russell Brunson

Stock traders can trade on their own account, called proprietary trading, or through an agent authorized to buy and sell on the owner’s behalf.
Trading through an agent is usually through a stockbroker. Agents are paid a commission for performing the trade.

Major stock exchanges have market makers who help limit price variation (volatility) by buying and selling a particular company’s shares on their own behalf and also on behalf of other clients.