What is a Second?
Seconds are perfectly usable full skeins, but with one or more major flaws. This flaw could be a giant slub, a large under-twisted bit, an over-twisted piggy tail, or perhaps a break in the ply. All of these will require that you break the yarn and reattach to continue with your project. In addition, some skeins will not have a ball band, depending on where in the production process the flaw was discovered.

Side note: Every project I make with my yarns is from the seconds pile, so don't be afraid. Grab yourself some great yarn at a discount!