Periodization Training

I went on the web to see if I could find points to support this post, but I was surprised to find a plethora of false / overly complicated information. That means, that anyone who doesn't work in the industry looking to learn about this style of training could have a difficult time, given the calculations of weights vs. 1 rep max, body weight, etc.

I'm going to make it really easy for you.

Periodization training is a way of dividing types of training up into... periods. It is good to stick to a particular workout / style of training for about 4-6 weeks at a time. This doesn't mean necessarily repeating all the same exercises over and over (especially if you're with a trainer, they will know which ones to stick to for the style and your goals), but keeping the repetitions around the same number, and the load at approximately the same difficulty. For example, many divide periods into programs for endurance, for strength, and for power. It's good to "master" each zone, and then move onto the next when its demonstrated its excellence. 4-6 weeks is usually the right amount of time for the body to excel at the program, and then to be shocked back into progressing / transforming with the next change.
For endurance, you will want to keep your weights lighter so that you fatigue between 12-15 repetitions (sometimes even 20-30 repetitions, depending on the sport for which you train, or on the client's body type). For strength, the norm is between 8-12 repetitions. For power, you need bursts of strength in smaller spurts, and repetitions can fall between 3-6 or so.
It is also important to incorporate plyometrics for power and stabilization.

Periodization is important for athletes because each of these components need to be fine-tuned to succeed at the activity. In running, for example, one will need power for the start and for the sprint in the finish, endurance so that the muscles can last the race with out fatiguing, and strength so that the muscles don't fatigue before the heart, and that they protect the joints, and to support the metabolism in general.

Periodization training is also important for the non-competitive individual, as it leads to consistent progress. The body does hit plateaus once it's mastered a certain style of training. Variety is the key to success, because it contributes to overall conditioning (and with it, you'll know your body can handle whatever life throws at it!). The body frequently needs a jolt of surprise to get its butt moving again, transforming the physique and being presented with challenges to overcome.
I know we're always wishing that the challenges in life would cease, but really... then we would exist at a standstill and never change, never progress. How gross! Imagine you were to have stayed where you were two years ago? I'm not talking age, I'm talking wisdom, development... I certainly don't want that!

Remember - this is a guideline. If you need specific advice catered to you, recruit the help of a professional to get more bang for your buck.

Ultimately, change it up. Whether your goal is to play for Team Canada, or simply to feel tight, strong and invigorated.