Woolen yarns are spun with the fibers going every which way. This causes the yarn to be light, airy with small fiber ends poking out. They are also warmer because of the airiness. Worsted yarns are spun with all the fibers aligned, making a smooth and dense yarn with more sheen and drape. Many commercial yarns are spun with the worsted style.
I once had it explained very simply: Woolen spun yarns are like bedhair in the morning - going every which way, and worsted spun are like hair after it's been combed.
Examples of Woolen Spun:
Cascade Eco+ Merino DK
Cascade BFL
Examples of Worsted Spun (not a complete list):
Malabrgio Rios
Cascade 220 Superwash