Singles or Plied?

With new yarn trends popping up each month, it's safe to say there's an overwhelming amount of choices when it comes to fiber arts - these days. There are SO many options.. but, everyone's current favorite, is -- by far hand-dyed yarns. I've been dyeing and spinning yarn for 6 years.. so, I know a little bit more than the average knitter/crocheter. Have you ever wondered the difference between a singles yarn and a plied yarn? I'm going to have both available in my update this week, so I thought now would be a great time to bust out a little bit of yarn-y knowledge for ya'll..