The following paragraphs will briefly describe what each of these terms mean.
Periodization
Periodization is a term used to describe the breakdown of a training program into periods, or phases. The idea behind periodization is to constantly present a new training stimulus to the body to force it to repeatedly adapt to this new stimulus.
If a new training stimulus is not presented, the body will adjust to the training and performance will not continue to improve and will eventually decline. On the other hand, if a new type of stimulus is presented too soon, the body won't have had enough time to react and adapt, and again, adaptation will decrease.
This is the reason you should see programs last only about 4 weeks. Trained athletes can adapt much quicker to new programs, and should consider changing programs more frequently. Athletes newer to training can continue to make gains for much longer on the same program. This is the reason you see many people "plateau" in their training. They aren't taking advantage of the principle of periodization.
Periodization is accomplished by varying the training intensity and volume during certain phases to continue to place stress on the body that it isn't used to.
Specificity
Specificity refers to training specifically for a desired outcome. You will train differently to build muscle mass than you would to improve performance. You would train differently if you wanted to run a marathon as opposed to play football. There are different strengths, movements and physiological requirements that need to be more developed in certain activities than others. Everyone has things that that they should be training to improve, whether it's health-related or performance-
Overload
This is the most important principle behind any training program. Overload can be substituted for the word stimulus or stress in the description of periodization. To create an overload, the body must be made to do things it is not accustomed to. This places a stress, or shock, on the body. If the body is not given an overload, it will not adapt. If there is no adaptation, you will not get any stronger or any more conditioned than you already are. In short you will not see results. Keep this in mind during your training. Always push yourself to improve on your last workout.
Overtraining
Overtraining is when fatigue or a decrease in performance arises from excessive training and a lack of proper rest and recovery between workouts. Overtraining is an injury. In most cases, overtraining is caused by "too much, too soon", or simply training too often and not allowing the body the proper amount of rest from an intense workout.
Some symptoms of overtraining may or may not include: Increased resting heart rate, increased resting blood pressure, decreased sports performance, slower recovery after exercise, weight loss, decreased appetite, decreased desire to exercise, increased irritability and depression, and increased incidence of injury.
