The days when soccer players could focus on drills, practice, and be competitive are gone. Modern soccer players are like any other pro athlete, the complete package. Soccer players have to strength train throughout the year. If all they do is lift weights like a bodybuilder, with the usual three sets of eight reps, they will go nowhere. Strength training for soccer works in three broad categories:
Maximal strength training
Maximal (or absolute) strength is the amount of strength that an athlete can produce at any given time. For example, a person lifting 150lbs has less maximal strength than someone who can lift 200lbs. Since maximal strength does not make any allowance for time or time under tension, its only part of the training regime to produce more power.
Muscular power is a function of maximal strength and speed. When you increase any of the two, you produce more power. Increase both, and there will be a significant increase in muscular power. Another way to increase muscular power is through plyometric training. Plyometrics, also known as jump training, consists of explosive movements to increase muscular power by training both at once.
Building muscular endurance involves something similar to circuit training. This training is continuous light weight strength training with limited rest periods. With muscular endurance, muscle groups will perform at high intensity, repeatedly for much longer.
Follow the three categories of training above, and you will improve both upper body and lower body strength training. This training is directly applicable to soccer on the field.