When airlines refer to your "carry-on bag," they are referring to smaller suitcases, duffel bags, and weekenders that fit in the overhead bin space onboard. "Personal items," in airline industry terminology, are small enough to fit under the airplane seat, like travel totes and purses, laptop bags, diaper bags, and briefcases or backpacks.
These differences may seem subtle, but there are limitations regarding both personal items and carry-on luggage. Check out our guide to personal items, and see major airlines' restrictions on these under-the-seat bags.