threads and their applications

[C&R]

Everytime I go into a fly shop and look in the materials section, I see all of these different types of threads and don't know what their applications are. I've been told to use only 8/0 and 6/0 UniThread for my trout flies. Where can I find information on what threads serve what purpose?

Also...

In the tying class I took last night, we made a no-name soft hackle wet fly. This involved tying on a strip of gold ribbon wax or something like that, wrapping it over the hook to cover it, and then wrapping a piece of dark red floss or something like it to make a segmented body. I found working with these two material difficult becuase they both had a tendency to fray. Can anyone tell me more about these and any tricks to make the tying easier? It was the first time I was working with them.