Due to inconsistent definitions of yarn weights and a bit of knitting history, some people assume that yarn ply has something to do with the weight of yarn you're working with. The truth is a lot more complicated than that.
Ply vs. Weight
Back in the good old days when most people were knitting with wool yarn, ply actually did mean something when it came to the weight of yarn. A ply was always a consistent size, so a 2-ply yarn was very thin, while an 8-ply yarn was much bigger.
There's no such thing as a 1-ply yarn, by the way. Technically, that's called a singles yarn. The singles are plied to create yarns of different thicknesses, but there's no longer any consistency as to the yarn's weight based on the number of plies.
A 4-ply yarn can be bulky or medium weight, while singles can be super thin or super bulky, or anything in between for that matter.
Standard Yarn Weight System
The old definitions of yarn weight as being a particular ply are disappearing in favor of the Craft Yarn Council of America's Standard Yarn Weight System. This system makes it possible for people from anywhere in the world to understand how thick or thin a yarn is because the system is based on numbers.
The system uses a scale from 1 to 6, with 1 being the finest yarn (also known as superfine) and 6, super bulky, being the biggest.
Ply Rules of Thumb
Should you see knitting instructions that call for a particular ply of yarn, however, these rules generally apply, based on the standards in the UK: