A Doctor of Philosophy (PhD) is a person who has demonstrated expertise in a particular field (thus, the "Doctor") and has made a significant and original contribution to the field in the form of new research, theory, etc (thus the "philosophy" part). Thus, a person awarded the PhD for a thesis concerning nursing theory/research is considered an "expert" nurse, and can best be thought of as a person who pioneers new developments and leads the profession in new directions (either as an educator, researcher, clinician, etc).
On the other hand, a Doctor of Medicine is a person who is considered an expert in the field of Medicine. An MD is awarded for satisfying the coursework requirements of a medical program, and while many people who have been awarded the MD will go into research etc, many are happy to remain in clinical practice (some medical programs only offer the combined MB BS, however graduates still adopt the title "Doctor" for professional recognition purposes - just a snippet of trivia for you...). Recently, many universities have introduced the DNSc (or Doctorate of Nursing Science), in recognition of the fact that nursing has evolved to the point where it has several subdisciplines in its own right and that there is now a need for a coursework program at the doctorate level as in medicine.
Thus, people who choose to study for the award of PhD in Nursing aim to further the development of nursing as a profession (generally). As many posters have stated, you can not compare the knowledge of a Doctor of Philosophy to a Doctor of Medicine (unless of course an MD studies for a PhD, in which case its a whole new bag of kettle crisps...) - nursing and medicine are two different disciplines that just happen to share similar goals.