In medical terminology a “sign” is something that can be seen or measured and a “symptom” is something that is felt by a person but can’t be seen or measured.
Let's take a look at a common problem, the stomach ulcer.
What are medical "signs" of ulcers?
A sign is something that can be seen or measured. So, for example, blood in urine can be a “sign” for an ulcer (or other problems like kidney disorders). Doctors can also stick a scope (a tiny camera on the end of a flexible tube) down the throat, peek around the stomach, and actually see an ulcer. A picture of the ulcerated stomach lining is a definitive sign of an ulcer. Doctors can also run a test to see if you have h pylori, a bacteria that is in the main culprit in most cases of ulcers. A picture of an ulcer or a positive screen for h pylori are both "signs" of a stomach ulcer, medically speaking.
What are symptoms of ulcers?
Symptoms are things that we can feel, but that can't been seen or measured from the outside. Stomach pain, nausea, and upset stomach are all symptoms of a stomach ulcer.
What are doctors looking for?
Doctors use medical signs to determine the best course of treatment. For example, if you present with a sore throat (your symptom) and the doctor looks for a sign to explain the symptom. The doctor runs a test for the strep bacteria. Whether or not strep bacteria are present will help the doctor decide the course of treatment, whether or not to prescribe antibiotics.
In order to diagnose a problem, doctors usually rely on a combination of your report of symptoms and their finding signs of a disorder. Sometimes medical problems, especially those related to pain, don't have reliable signs. For example, low back pain is a symptom. Sometimes, though, there aren't signs a doctor can measure or see to explain pain. This can be frustrating for both doctors and patients.
Just because there aren't observable signs to explain the sympton of pain does not mean that pain is not real.
It does mean that it is going to be more difficult for your doctor to determine the best approach to treatment.