A symptom is a physical sign or condition that does not generally occur in a healthy person. They are always a sign of an underlying disease. A diagnosis is the definition of the disease that is causing the symptoms seen in a patient.
For example, a body temperature of 98.6F is not a symptom (although it is a vital sign). A body temperature of 101F is a symptom, as is a body temperature of 95F. However, there are several conditions that can cause either symptom, in the case of a fever, anything from hyperthermia to any one of thousands of possible infections.
Unfortunately, the body has a limited number of symptoms, and few symptoms are unique to any one disease. However, most diseases have multiple symptoms. For example, measles causes both a fever and a distinctive rash.