The idea behind crossfit is to build mass in a leaner way, with a lot of weight loss built in. There is less emphasis on building huge muscles than on bodybuilding. That said, CrossFit has a competitive objective. It's like a decathlon, only with different exercises.
The combination of CrossFit-style workouts and bodybuilding allows you to improve your overall physical performance while gaining muscle gains. Both CrossFit and bodybuilding involve lifting and lowering weights, repeatedly, several times a week. The real flesh is in the differences. CrossFit is different from bodybuilding and weightlifting in that it's more accepting and encouraging.
At CrossFit, people usually exercise in groups and follow a WOD (workout of the day). CrossFit gyms are radically different from commercial gyms in that they don't have elliptical machines, treadmills, or weight training machines. Bodybuilders use machines in their workouts to include variations and isolate their muscles. Most bodybuilding gyms are filled with numerous machines to add variety to workouts and grow muscles.
Recent studies show that CrossFitters suffer no more shoulder injuries than other athletes, for example. CrossFitters don't usually target a specific muscle group, such as the biceps or chest, and prefer to follow a full body workout. All bodybuilders have a bit of CrossFitter in them (because when they get down to business, all bodybuilders get an intrinsic joy from lifting a heavy weight). Similarly, the first thing that comes to mind for a crossfitter, when debating with them about bodybuilders, are gross, vain and chicken legs.
The average recreational Crossfitter generally has a good amount of muscle with a low amount of body fat compared to the average person, while a professional Crossfit athlete has enough muscle to compete with a bodybuilder's physique. Every CrossFitter has a little bodybuilder in them (or else there wouldn't be as many CF Instagram accounts filled with black and white photos of chalk hands attached to shiny bodies amidst cleanses, thrusters, and muscles). In terms of smart looks, you can see that CrossFitters and bodybuilders look very different (see image above). Since work done over time is a big concern, CrossFitters will often look for shortcuts to improving performance without building the right foundation.
A bodybuilder will focus on strength and high rep work (to pump muscles), while a CrossFitter will focus more on speed, high reps, and endurance. While bodybuilders train to create their physique by developing muscle mass and symmetry, CrossFitter exercises to get fit. CrossFitters rely on functional movements in their training, which generally include dumbbell bars, kettlebells, and pull-up bars, and don't use many machines. CrossFitters also do group workouts in which a large number of people perform the same training as a “class”.
That said, if you can get a CrossFitter to do bodybuilding training, they won't be able to perform as well as the bodybuilder.