Measles is a highly contagious viral infection whose hallmark symptom is skin rash. The first signs, which occur 9-12 days after exposure, are fever, cough, runny nose, & red eyes. A few days later, spots in the mouth appear, then skin rash, that typically starts on the face & spreads down the body. Although skin rash is thought of as the main symptom of measles, respiratory symptoms & complications, including pneumonia, bronchitis, & croup, can cause more serious problems. Other complications include ear infection & diarrhea. In the US, measles has been almost completely eliminated by the MMR vaccine, but in other parts of the world it's still a serious problem.
Picture from PHIL/CDC (See Measles pictures), text paraphrased from several sources on list below.