The difference between and agent and a broker is that an agent usually sells insurance for a single insurer, while a broker sells insurance for any number of insurers.
In some cases an agent may actually be employed by the insurance company, although there are also agents who are self-employed.
If you're looking for details about a specific insurer's products, an agent may be the best person to talk to. However, if you're trying to comparison shop across multiple insurers, you may want to contact a broker.
Agents and brokers always provide their services for free, and earn commissions off of the policies they sell.