Sensitivity = I am going to explain the sensitivity of a radio. There are 10 stations in the air. If it can only pick up 7 of them, it is 70% sensitive. Therefore if there are 10 cases of a disease in a population, and only 7 can be picked up by the test, it is 70% sensitive.
Specificity = Of all that are free of the disease, how many actually test negative. Now we think of a machine which opens oysters that have pearls inside. We feed it 20 oysters. It opens 8 of them. 2 out of the 8 did not have pearls. We then went on to open all those that it did not open and found 3 pearls but 9 were without pearls. Therefore the specificity is 9/11 - i.e. of the total without the pearls (11), it correctly identified 9