Yarns have different weights. There is fingering, sport, dk, worsted, chunky, etc.
The different weights also have different plys. Plys are how many strands are in the yarn. There can be different plys for the same weight class; for example, there is 2-ply and 3-ply sport weight.
You use different weights for different things. You would use fingering weight for making socks.
Always work up your gauge sample before starting any project to make sure your gauge is correct. Otherwise, your project will be bigger or smaller than the directions state, and you might just run out of yarn! This is especially important when making a garment with a size.
By the way, I like Red Heart. It wears well and doesn't pill. And the price is right. Simply soft is soft, but has no "body" to it (it tends to "droop"). I love the TLC Amore yarn and would say it's a dk weight. Try to either hand wash and lay flat to dry to avoid the pilling.
