Not a doctor, but I believe the two terms are used essentially interchangeably.
I'm also not a doctor, nor am I a pathologist, but I've learned a bit along the way...so the usual caution applies to take everything you read off chat sites like this with a grain of salt. Mel, you already know that, I'm sure, since you are a regular poster and you have seen the wide variety of "knowledgeable" responses. It's amazing how many different responses to the same question show up here.
Nonetheless, I will put my own understanding of positive margins into meaningful, layman's wording...maybe more than you want/need to know.
For starters, note the term "surgical margins"; this is the outer edge of the tissue removed during surgery. During surgery, the surgeon attempts to removes the prostate and some normal tissue surrounding it...the surgical margin is what is around the outside of all of what was removed. The area of normal tissue is important because any stray cancer cells may be included in this. If the edge (or margin) of what was removed contains PC cells, then there may have been PC cells left behind in the corresponding spot in the prostate bed. The goal of surgery is to achieve a "clear margin", that is, to remove all the cancer...even if a little normal tissue is removed along with it.
The prostate and the other tissue that was removed are sent to the lab where the pathologist coats it with an ink. The ink coloration and penetration help the pathologist to determine how close any cancer comes to the edge. The ink stains appears different when PC cells are present on the margin.
Those patients with positive surgical margins are at an increased risk of cancer recurrence. The pathologist will note in his/her report the number and location of positive margins. Patients with more than one positive margin are more likely to have cancer recur compared to those with a single positive margin. Also, patients with a large area of positive margin are more likely to have recurrence of PC compared to those with a small area where the cancer just touches the edge. It is important to note that most patients with positive margins are "cured." Depending on the number and extent of margins, post-operative radiation may be recommended to decrease risk of recurrence.
You've probably heard discussion about the pros and cons of open and laparoscopic surgery having to do with this same concept. open surgeons use their fingers to feel the prostate (they certainly can't see much through all the blood); cancer cells are firm. In laparoscopic surgery, obviously the surgeon doesn't have the same tactile sensation, but instead has magnified visual cues to follow to improve outcomes; specifically the color of tissue, signs of inflamation, etc. In each case, an experienced surgeon would likely be better tuned-in to the tactile (open method) or visual (robotic method) cues, and would know to adjust the amount of normal tissue removed accordingly to increase the likelihood of a negative margin. Important reason to have an experienced surgeon...either open or robotic (laparoscopic).
Hope this helps explain the big picture...