50 wt 2 ply is thinner then 50w 3 ply. These are the common sizes for piecing. Lower sizes means thicker thread and they are best for machine quilting when you want the stitches to show. Very thin thread 60w and higher is great for bobbins and for machine quilting when you want the stitches to sink into the fabric and many use the thin thread for piecing. Example: Aurifil cotton 50w is 2 ply. Connecting Thread Essential cotton thread is 50w 3 ply.
For 50w 2 ply and higher weight thread you need a small size needle, I use size 80 or 70. 50w 3 ply thread is generally used with a 90/14 size needle.
Thread is an education all in itself. Here is a good place to start and also a great brand to buy to start learning about thread:
http://www.superiorthreads.com/media...HOME_12-11.pdf