A will is not the only way to divide assets. You may also obtain those assets if you're a beneficiary listed on a financial account. For instance, perhaps your parents took out a life insurance policy and listed you as the beneficiary, meaning that the policy pays out to you when they pass away.
A person who is disinherited is typically either an heir who would naturally expect to inherit money but was left out of the will or another person -- even if not related to the individual who passed away -- who was named in a prior will and then removed from the most current will. For instance, a person could initially put a close family friend in the will, lose touch with them over the years, and then remove them from the next will. In other cases, people cut out one child while leaving everything to other children.