A year-round training program ideally incorporates weight training, aerobic activities, climbing-specific exercises, and cross-training. The quantity and intensity of training depend on the season. For example, winter is traditionally a time when rock climbers hit the gym to build a base of strength for the upcoming year with weight training.
As spring approaches, the mix becomes more climbing-specific as actual climbing begins. Summer is a time of maintenance with light training and running between climbs. Fall is sometimes a great season for a climbing road trip, so training becomes more climbing-specific to help reach a goal.
For most climbers, training for rock climbing means doing lots of climbing. In many ways, this is the best approach, because nothing trains you for the real thing like the real thing. Outdoor rock climbing can be used as a vehicle for training, except that it rarely offers consistency. Weather can be poor, climbing can be too easy or too hard, and too many factors are uncontrollable.
Indoor climbing, in a gym or on a home wall, has changed that by providing a controlled environment. Now a climber can plan exactly when and how a climbing-specific workout will occur. The result is that it is easier than ever to quickly learn how to climb well and get fiendishly strong.
The downside is that it is also possible to overdo it way too quickly and suffer chronic overuse injuries. To be effective and smart in a gym or other controlled climbing environment, approach the experience with a plan. Start with a warm-up, then focus on endurance or difficult climbing such as trying a hard redpoint.