Each is important with respect to attaining the desired results. For example, if muscular endurance is the objective, the training will be different than if muscular strength is the objective. Mode of training is the method of training. Free weight training versus weight machine is an example of mode of training. Finally, using an isokinetic machine as oppose to an isotonic weight machine is an example of mode of training.
Resistance refers the amount of weight used.
The heavier the weight the greater the resistance. Sets refers how many time one does a particular weight lifting exercise. Repetitions are the number of times a weight is lifted in each set. To train for strength, 3-12 repetitions are recommended. If muscular endurance development is the objective, each set should consist of more than 12 repetitions.