the numbers on a spool of thread are your (keys)
if your thread says---- 50/3 it is a 50 wt thread made with 3 plys-
if it says 40/2 it is a 40 wt thread with 2 plys---the higher the number the thinner/finer the thread-
12 wt thread is alot like string- 100wt thread is very fine (generally silk)
the ply is the number of (threads) spun together to create the thread- generally 2 or 3- but sometimes more or less
generally we use 40 or 50 wt thread for most piecing/quilting applications-
60 wt is often used as a bobbin thread.
visit Superior Threads- they have FREE charts you can print off which tell you the differences in the threads- the sizes/weights- the needles to use with each thread- and what they are generally used for- it is a very good resource- they have alot of invaluable resources - including Thread University.