CrossFit has at its very core three fundamental pillars that prop up the very nature of its being. In order for your body to continue to develop you must not subject it to the same thing over and over again. Your body will grow into the nature of its demands. Ask nothing of it and it will give you nothing in return. Ask the same thing all the time and that's what you get. However, if you make ever changing demands on your body your body will adapt to those changes and grow to accomodate itself to them. These demands must change not only in kind but also in ever increasing level of difficulty. Therefore the first of the three pillars of CrossFit is Variety.
Pillar number two has to do with the relationship your exercise has to do with the way you interact with life. In order for your exercise program to be effective it needs to simulate athletic movements. How often in your athletic endeavors do you accomplish a task with a pec deck type movement or for that matter a leg curl? Compare those movements with the power clean or the thruster - a barbell front squat to overhead push press - these movements are incredibly functional and can move large amounts of weight through a long line of motion at great speed. The second pillar of CrossFit is Functionality.
The final pillar determines not only the rate at which you can expect your results to come to you but also the height of your achievement. Hard work is rewarded with results just as laziness is self-punishing. Pedal along at a comfortable heart rate on a recumbant bike as you thumb through a magazine and your rewards will be few, but mark your training with a Variety of Functional exercises executed at very high Intensity and you will be rewarded with improvement in all 10 athletic skills. Intensity is uncomfortable, intensity is even intimidating but intensity is also where the results are. The final pillar of CrossFit is Intensity.