A  broker is an individual or a company who facilitates financial transaction execution on behalf of a trader. They basically act as sales agents. 

There are mainly four types of broker in financial trading

Stock broker : Stock broker manages and executes the buying and selling of securities for the traders.
Forex broker :Forex brokers buy and sell  currencies for the traders.
Full-service broker :Full-service brokers are financial advisers and offer more services for traders / investors  ( on top of buying and selling ). Example tax advice, research, financial  planning. 
Discount broker : Discount brokers offer lower commission for executing the trades for the traders. The more trades you execute with them, the lower the cost.

Practice Brokers – Beginner