IMHO, this is the best website out there to learn about threads....look for the links to explain all of them plus the needle size recommended. There are a number of tutorials also as Bob goes all over the US giving his class of threadology.
Yes, they're somewhat expensive but I love King Tut and many of his others. If these threads are more than you want to pay you can still gleen so much information to help you to learn about threads.
http://www.superiorthreads.com/