The word "ginger" is often used to describe any person having red hair; However, a person is only a true ginger if he or she has:
1. Red Hair (any shade)
2. Brown Eyes
3. Pale skin
Depending on the circumstances and location "ginger" is being used, it can be extremely offensive due to the inplication that gingers have no souls and are lesser peoples. However, it is generally meant to be used in a light-hearted manner.
Bob: "John, look at that ginger over there! She clearly has no soul."
John: "She may not have a soul, but she has blue eyes, so shes not really a ginger."