Cooling Efficiency in computers is impactful in every aspect of them.
All computers need to be cooled. Circuits produce heat when current is running through them. This heat will eventually cause the breakdown of circuits in multiple ways, so the heat must be taken away. There are many different methods of taking the heat away, air is the most common for everyday use, followed by water to keep chips cooler and quieter, and then harnessing the power of phase changes of chemicals to push chips to their highest potential. The cooler a chip is, the longer it will run, the more efficient it will be, and the further you can push it. This means an understanding of cooling is very important to an understanding of efficiency, especially when it comes to computers.
Heat and Circuits
Circuits are based on voltage and resistance. For a circuit to run it needs a conductor and a difference in voltage. Every conductor has a resistance, and that with the difference in voltage determines the current. Within circuitry some energy is turned into heat, and the amount of energy that turns into heat is determined by this resistance. Lower resistance actually causes more energy to be released by a circuit. (The equation for the power released is based on resistance times the square of the current).
Heat has a major impact on circuits. Too much heat can stress computer component. This is for a few reasons. Heat expands the materials the circuit is made out of, and can eventually cause them to break. This is especially bad when different materials expand at different rates. Heat can also change resistance, which can impact a lot of parts. A change in resistance can cause distortion of signals and loss of linearity. For computer components an increase in heat also causes an increase in power consumption, which compounds the problems and decreases power efficiency. 
Forms of Cooling
All the heat in a circuit must be carried away in some way. CPUs for example, can take hundreds of watts of energy in normal function. There are three main ways of carrying this away, are air, water (also known as liquid cooling), and phase cooling is not common outside of competition. Within these methods of cooling there is usually a few different elements that are in common. The first is the idea of a “heat sink” or somewhere that the heat from the electronics goes to temporarily to hold it temporarily. These heat sinks can usually hold much more heat energy than the electronic components.After the energy is taken from the electronics it is then radiated away from the heat sink. Most modern computers require this to even turn on. This is because there is too much energy for an average CPU to deal with alone, even when it sits idle.
Air cooling is the simplest, and the most common. In air cooling the heat sink is typically a large piece of metal that is made out of copper and/or aluminum alloy. The most often scenario is that a fan is placed on top of the heatsink to cause airflow, which then carries off the heat. Heat sinks are typically made to have the most possible surface area, so that the air can carry away the most possible heat. Some CPUs (like ARM for example, which is most commonly used in mobile devices like phones), do not require a fan. These are then cooled passively and the heat dissipation of the extra surface area provided by the heatsink is enough. 
Air cooling will be seen in the most amount of computers. This is because it is so simple. These are given out with the majority of CPUs, and are almost always attached to any external GPUs. Most cases and their surrounding areas for PCs have sufficient airflow to allow cooling, as space is not often a major concern. When space does become a concern, in laptops and server racks for example, fans must be turned way up. This does cause some issues, noise and power consumption can increase heavily. To keep a server farm running the air usually has to be cooled heavily before it ts brought in. As space becomes a bigger concern the amount of energy required increases quickly. The maximum accepted density is about 356 watts per square foot. This escalates to the point that specialised air channels and hot areas must be created. These can require specially build rooms and may need to be turned off when maintenance is done by humans.
The next most common form of cooling is water cooling. In this form of cooling the water is actually the heat sink. This is because water can hold much more heat than most other chemicals we use here on Earth, and can remove heat 50 to 1000 times faster than air. The problem is that water and electronics don’t mix well, at least directly. The water is typically carried in tubes, with a special clamp-on to the cpu that serves to transfer heat between it and the water (because touching the cpu directly can cause problems). The great thing about water cooling is that the water can be moved. A typical water loop can include a pump, radiator, and reservoir. The pump moves the water, while the radiator holds some of the heated water to allow it to cool off, and the reservoir holds extra water. Water coolers are typically much quieter, and are able to hold and radiate much more heat than air coolers. The reason they are less used is because they require more expensive equipment and are more difficult to set up.   
Water cooling is becoming more common in both personal computers and in the server space. This is often used when parts are pushed to the limit for long periods of time. Since heat can damage parts over time, overclocking on liquid cooling can help get the most out of parts in both clock speeds and longevity. Since coolers can be re-used pushing a cpu to the limit until it comes time for an upgrade can get you more for your money. This also applies in the server space where the amount of server failures has been observed to be reduced in centers which use water cooling, by as many as 5 times less. In the server space water cooling also serves a different use. In a typical home computer all of the extra parts that a water cooler has can increase power consumption over a fan, but in the server space a lot of extra parts would be needed already. This means that since water cooling is more efficient, water cooled servers are as well. This reduces the power consumption of the cooling by nearly 20%. Density is also increased heavily. Servers can be pushed together to the point of having a heat output of thousands of watts per square foot, opposed to the just hundreds of watts per square foot air cooling can pull off. This saves massive costs when server areas are in crowded areas like cities. When area is expensive water cooling can be as much as a one third the cost, but when area is no problem air cooling is often a little cheaper.
Phase cooling is by far the least common. The basis of phase cooling is not like the other two, instead of using a heat sink to transfer waste heat to the environment it instead lowered below the temperature of the environment. This is able to be done because of the properties of changing states of matter. As a liquid turns into a gas it expands and cools. This can then allow it to absorb heat from the CPU much faster and for longer, than the other methods. This also usually requires tubing, a pump, and a reservoir. However, it also requires something to compress the gas back into a liquid, so that it can be used again. Some methods of this are actually completely open, and gas is manually put in. This is often far more expensive than any of the other options. 
Phase cooling is the least used out of all of these for both home use and servers. Phase cooling can cost hundreds to thousands of dollars, and typically reduces the lifespan of components it cools. Overclocking competitions are the only place they seem to regularly be used. In server environments that is bad news, but there is testing going on in that area. “Full-immersion” technology involves putting the whole (or most) of the computer within a liquid. As it boils it takes energy away. The liquid eventually condenses and re-joins the liquid by the computer, allowing it to cool everything again. There are also versions of this that do not include phase changes.
Each type of cooling has its own use case. This means learning about all of them is important to building a functional system. My next post will probably be about improving overall efficiency in cities or something