More exactly, how do they differ from wet flies?

Most of the information I?ve found deals more with historical or instructional aspects of nymphing. What I?ve learned from my searching and talking with other FFers, is that wet and nymphs are very similar except wets tend to be winged flies while nymphs are not. Also, it seems how they?re fished is different too. There is lot of stress placed on presentation in nymphing. For example, line control, drift, etc. Is this correct? I?ve never fished with wet flies, but wouldn?t presentation be just as important when using wet flies?

I started thinking about this question when a friend mentioned that a leech fly is considered a nymph. I know there are tons of variations of leech flies, but I was really surprised to learn that they tend to be nymphs, as well as San Juan worms and wollybuggers (although most people consider them streamers?). It?s hard to believe those could be considered nymphs.

I hope people can set me straight on this question or direct me to useful sites/books that can explain to me what exactly the difference is between these two types of flies.

By the by, in my search for finding an answer, I searched the FAOL site and discovered (keep discovering) the plethora of useful information on this site. I especially liked the chapter, Fishing Nymphs, from Fly Fishing 101 section.

Has anyone come across any useful pages within FAOL that could help me answer this question?

(Okay, I plead ignorance...but that?s why I?m here, right?)

Thanks ahead for any info.