The key difference between desktop computers and laptops is the compromise between performance and portability. You will almost universally get better performance per dollar from a desktop machine, while a laptop trades horsepower for a compact chassis that's easy to take on the go. While there certainly are laptops built around mighty hardware capable of rivaling all but the most high-end desktops, they tend to be absurdly expensive (and often approach the 'musclebook' category that sheds some portability for more powerful components).