The previous answers are all on the mark. What I can add is the research seems to be clear in suggesting that in order to become noticeably “stronger” it is important that the targeted muscles be taken to temporary failure during the exercise, typically with a weight that can be taken to 8-12 RM. This applies to a general fitness strength training program. Of course, for performance athletes (such as the bodybuilder in LaRue’s answer) many factors need to be considered when designing the athlete’s strength training program.
Also, as I’m sure you know, including a complete stretching program in conjunction with strength exercises is most important. It appears that stretching following the strenth workout is the most effective.