The National Association of Social Workers (NASW) is a treasure trove of great information for those considering the social working career and professionals alike. However, most of the information you really want to read is now only accessible to those who are paying members of the organization, and also may need to pay additional fees for career field specific articles.
NASW's short social work career overview, brief explanations of varying career fields, and a short quiz to help you determine what type of social worker you might be.
NASW provides a great cursory overview of what social work is all about in general. Additionally, they provide articles about the different types of social work, typical social worker values, and an interesting “quiz” to help you learn what type of social worker you might be.