Nearly all of the energy a computer consumes is converted into heat. That heat is the result of the resistance between the electrical current and the components it travels through. This is known as Joule heating, and is how toasters and space heaters work. In the case of computers, however, it's not a good thing.
As I write this, my laptop's CPU is 50°C, despite being about 90% idle. All that generated heat has to go somewhere. This is typically accomplished by a combination of fans and heatsinks. They can only accomplish so much, however, and higher-clocked CPUs require water-cooling, or in extreme experimental cases, liquid nitrogen-cooling.
The maintenance of water-cooling isn't reasonable for the average person, or for mobile devices. Most tablets and phones don't even have fans, and rely entirely on passive cooling. Recent advances in CPU design have focused on energy efficiency, both to reduce to heat generated and to improve battery life.