According to the US Department of Labor, Occupational Outlook Handbook article "Human Resources, Training, and Labor Relations Managers and Specialists"
Every organization wants to attract, motivate, and retain the most qualified employees and match them to jobs for which they are best suited. Human resources, training, and labor relations managers and specialists provide this connection. In the past, these workers performed the administrative function of an organization, such as handling employee benefits questions or recruiting, interviewing, and hiring new staff in accordance with policies established by top management. Today's human resources workers manage these tasks, but, increasingly, they consult with top executives regarding strategic planning. They have moved from behind-the-scenes staff work to leading the company in suggesting and changing policies.
In an effort to enhance morale and productivity, limit job turnover, and help organizations increase performance and improve results, these workers also help their companies effectively use employee skills, provide training and development opportunities to improve those skills, and increase employees' satisfaction with their jobs and working conditions. Although some jobs in the human resources field require only limited contact with people outside the human resources office, dealing with people is an important part of the job.
There are many types of human resources, training, and labor relations managers and specialists. In a small organization, a human resources generalist may handle all aspects of human resources work, and thus require an extensive range of knowledge. The responsibilities of human resources generalists can vary widely, depending on their employer's needs.
There are too many specialties to list due to space restrictions. You should see the Handbook's entire article to find the scope of the various specialties, educational requirements, nature of the work, and median salary levels. http://www.bls.gov/oco/ocos021.htm