There are several references for you to check out to answer this question. A good overview of the profession is provided in the Bureau of Labor Statistics Occupational Outlook Handbook. You may want to contact the National Association of Social Workers for more information. In addition, the National Association of Social Workers has a description of the social work profession on its website.
The New Social Worker magazine offers many good articles for prospective and current social work students. Many occupational outlook editions of common periodicals also discuss what social workers do (although not always accurately).
Other helpful resources are books: So You Want to Be a Social Worker, by Helen Perlman, and What Social Workers Do, by Margaret Gibelman (this book is also available through NASW Press). Of course, nothing beats a one-on-one chat with a practicing social worker. Look in your local yellow or blue pages to find social workers and agencies near you.