Blade servers are the latest development in the history of the different types of server. Slim and compact, just like a blade, they slide vertically into a specially designed chassis and are unusual in that they share certain elements of the hardware with other blade servers in the chassis for the purposes of efficiency and cost reduction. Blade servers, for example, use a single feed positioned on the host compartment.
In short, blade servers will give you much greater processing power, take up less space and use less energy than other forms of server used for the same purposes.
Of course, blade servers are also more expensive than the other two types of server, but the level of performance is obviously beyond compare.