CrossFit is a strength and conditioning program. In its simplest definition: constantly varied, functional movements, performed at high intensity. A functional movement is an exercise that we as humans perform everyday. The squat: moving from a seated position, to a standing position. The deadlift: picking up an object from the ground. The press: taking an object and putting it over head.
The flip side to that coin would be non-functional movements. Exercises that serve no purpose in the real world: tricep extensions, leg extensions, leg curls etc. Notice anything about these three exercises? There's two important things to note. You will only find these exercises being performed in large commercial gyms, where you're typically treated like a number and not an individual. You will never see these exercises being performed in a CrossFit gym. CrossFit focuses on compound (multi-joint) movements that elicit the greatest physiological and metabolic response. English? Growing stronger, becoming leaner, and ultimately getting fitter.
The CrossFit program is designed to train individuals to be prepared for any physical task. This concept is referred to as G.P.P: General Physical Preparedness. Sprinting, biking, weight-lifting, throwing, climbing, jumping, rowing, and even combat, are all forms of physically demanding tasks.