Cardio is exercise that gets your heart rate up like running, jogging, or bicycling, strength training helps your body to build muscle and the muscles help to support your bones and joints example would be weight lifting or circuit training, flexibility helps to stretch your muscles and tendons and will help you to prevent injuries yoga is a good example. Also it should be noted that many of these examples help the other fitness categories for example yoga and biking are also good strength training exercise.
Cardio exercises are meant to raise your heart rate; examples would be cycling, running and jogging. Strength training is meant to help your body build muscle with weight lifting being a prime example. Flexibility helps you stretch muscles in your body; stretching and yoga are good examples.