In an agency relationship, the agent is the person who is acting on behalf of the principal. By the definition of agency, the agent is acting on behalf of the principal and subject to the principal's control. The agent has the power to affect the contractual obligations of the principal to the extent the agent has actual authority or apparent authority. The fact that the agent is acting on behalf of the principal gives rise to a fiduciary duty of the agent to act in the best interests of the principal.
There are many different types of agents. One of the most important distinctions is between agents who are employees (i.e., who have an employer-employee relationship with the principal) and those who are non-employee agents. Non-employee agents are often described as independent contractors to distinguish them from employees.
It is also important to note, however, that not all independent contractors are agents. In many cases, one hires an independent contractor to perform a service without making that person an agent. For example, if you hire a lawnmowing service to mow your lawn each week, that service will be an independent contractor, but not likely an agent. This is because the service is not expected to act on your behalf in dealings with third parties.