Something altered is changed. Your recently altered appearance, including a new short haircut and stylish new clothes, might surprise your grandparents when they pick you up at the airport.
Altered is an adjective that describes anything that's been changed or modified. You might need altered clothing after you succeed at your diet, or feel like you are in an "altered state" after you've been hypnotized. Altered behavior is behavior you have changed, like the altered habits of your housebroken dog. Altered comes from the Medieval Latin alterare, or "to change."