Ply: This is a strand of yarn. You'll often see threads described as 2-, 3-, and 4-. The above image depicts a 3-ply thread.
Core spun yarn/thread: This type of thread is composed of a "core" of continuous filament polyester that is wrapped with either cotton or polyester yarn, like a cocoon. These combined threads provide qualities from both fibers, with the outer fiber providing the finished "look." For instance, a polyester core with wrapped cotton has the strength of polyester, but the finished look of soft cotton. This creates a stronger, more durable thread. You commonly see this type of thread used in the construction of Jeans.
Corded Thread: When multiple "threads" are twisted together, they create a "corded thread." These are commonly described as 4-, 6-, and 9-ply threads. As you can imagine, a corded thread will be stronger and thicker. You commonly see this used in leatherwork, shoe-making, etc.
Thread Twist: The direction the thread is spun defines its twist. It can either be twisted in a 'Z'direction (left twist) or an 'S' direction (right twist). A 'Z' twist is suitable for single-needle sewing machines. An 'S' twist thread isn't commonly used in home sewing machines.