Resistance training is training your body to work for longer periods of time. In resistance training you make your muscles work for long periods of time so that they are accustomed to work for long. This type of exercise is very good for your health since it makes all of your muscles stronger and accustomed to hard working.
Flexibility exercises are a type of exercise that make your ligaments stretch further. This is very beneficial because with a high flexibility you can do much more stuff such as making your body reach positions it wouldn’t reach otherwise.
A good exercise session is one in which you do all of this type of exercises in moderate amount. A good exercise regimen could be starting with flexibility so you can stretch your muscles and get them ready for exercise. Then you could do some cardio training by running on a treadmill for about 30 minutes and finishing with some resistance training so your body finishes its exercise. After you are finished it is usually good to do some flex so your muscles are in its normal position again.