The Specificity Principle simply states that training must go from highly general training to highly specific training. The principle of Specificity also implies that to become better at a particular exercise or skill, you must perform that exercise or skill. To be a good cyclist, you must cycle. The point to take away is that a runner should train by running and a swimmer should train by swimming.
While there may be other 'principles' of training you will find on the web and in text books, these 6 are the cornerstone of all other effective training methods. These cover all aspects of a solid foundation of athletic training. Once put together, the most logical training program involves a periodized approach which cycles the intensity and training objectives. The training must be specific not only to your sport, but to your individual abilities (tolerance to training stress, recoverability, outside obligations, etc). You must increase the training loads over time (allowing some workouts to be less intense than others) and you must train often enough not only to keep a detraining effect from happening, but to also force an adaptation.
Also see: The Principles of Sports Conditioning