Meta-ethics

"In philosophy, meta-ethics is the branch of ethics that seeks to understand the nature of ethical properties, statements, attitudes, and judgments. Meta-ethics is one of the three branches of ethics generally recognized by philosophers, the others being ethical theoryand applied ethics. Ethical theory and applied ethics make up normative ethics. Meta-ethics has received considerable attention from academic philosophers in the last few decades."

Here's a nice, brief, clear introduction to Meta-ethics.