Personal item: your personal item must fit under the seat in front of you. It cannot be larger than 14"H, 18"W, 8"D. These are usually purses, totes, computer bags, briefcases, small backpacks, and diaper bags!
Carry-on bag: your carry-on bag must be able to fit either underneath the seat or in the overhead bin. It cannot be larger than 24"H, 16"W, 10"D (including handles, wheels, and straps) and no heavier than 35 pounds. These are usually large backpacks, duffel bags, and small suitcases/wheeled bags!