The main difference is that an insurance agent is a direct representative of the provider of the insurance policy being offered to you, their responsibility is to sell the products from those insurers and no others. They may be a multi agent where they provide covers from several providers, but they continue to only sell products from those insurers.
An insurance broker on the other hand is able to access many different insurers (excluding those that only sell direct to the public), matching the customer’s needs to the insurance policy and negotiating changes that may be required. Insurance brokers are engaged by the customer and are working on their behalf to source, negotiate, maintain, renew and assist in the claim process. An insurance broker is an advocate for the client, not the insurance company.