Blade server

IBM HS20 blade server. Two bays for SCSI hard drives can be noticed in the upper left area of the image.
Blade servers are self-contained computer servers, designed for high density. Whereas a standard rack-mount server can exist with (at least) a power cord and network cable, blade servers have many components removed for space, power and other considerations while still having all the functional components to be considered a computer. A blade enclosure provides services such as power, cooling, networking, various interconnects and management - though different blade providers have differing principles around what should and should not be included in the blade itself (and sometimes in the enclosure altogether). Together these form the blade system.
In a standard server-rack configuration, 1U (one rack unit, 19" wide and 1.75" tall) is the minimum possible size of any equipment. The principal benefit of, and the reason behind the push towards, blade computing is that components are no longer restricted to these minimum size requirements. The most common computer rack form-factor being 42U high, this limits the number of discrete computer devices directly mounted in a rack to 42 components. Blades do not have this limitation; densities of 100 computers per rack and more are achievable with the current generation of blade systems.
Server blade
In the purest definition of computing (a Turing machine, simplified here), a computer requires only;
memory to read input commands and data
a processor to perform commands manipulating that data, and
memory to store the results.
Today (contrast with the first general-purpose computer) these are implemented as electrical components requiring (DC) power, and in operation produce heat. Other components such as hard drives, power supplies, storage and network connections, basic IO (such as KVM and serial) etc. only support the basic computing function, yet add bulk, heat and complexity, not to mention moving parts that are more prone to failure than solid-state components.
In practice, these components are all required if the computer is to perform real-world work. In the blade paradigm, most of these functions are removed from the blade computer, being either provided by the blade enclosure (e.g. DC power supply), virtualised (e.g. iSCSI storage, remote console over IP) or discarded entirely (e.g. serial ports). The blade itself becomes vastly simpler, hence smaller and (in theory) cheaper to manufacture.
Blade enclosure
The enclosure (or chassis) performs many of the non-core computing services found in most computers. Non-blade computers require components that are bulky, hot and space-inefficient, and duplicated across many computers that may or may not be performing at capacity. By locating these services in one place and sharing them between the blade computers, the overall utilisation is more efficient. The specifics of which services are provided and how vary by vendor.
Power
Computers operate over a range of DC voltages, yet power is delivered from utilities as AC, and at higher voltages than required within the computer. Converting this current requires power supply units (or PSUs). To ensure that the failure of one power source does not affect the operation of the computer, even entry-level servers have redundant power supplies, again adding to the bulk and heat output of the design.
The blade enclosure's power supply provides a single power source for all blades within the enclosure. This single power source may be in the form of a power supply in the enclosure or a dedicated separate PSU supplying DC to multiple enclosures [http://h18004.www1.hp.com/products/quickspecs/12330_div/12330_div.html]. This setup not only reduces the number of PSUs required to provide a resilient power supply, but it also improves efficiency because it reduces the number of idle PSUs.
Cooling
Operating the electrical and mechanical components of a computer produces heat, which must be displaced to ensure proper function of all these components. Fans are the most common method used to remove this heat in computers, but these add bulk and more moving parts. The blade enclosure typically provides fans to remove hot air from within the blades.
A frequently underestimated conflict in the design of a high-performance computer is the trade-off between design for density and the ability of the fans to move hot air away from the system. Since much of the bulk of a traditional server is removed from a blade, it can be designed to allow for excellent airflow.
Networking
Computers are increasingly being produced with high-speed, integrated network interfaces, and most are expandable to allow for the addition of connections that are faster, more resilient and run over different media (copper and fiber). These may require extra engineering effort in the design and manufacture of the blade, consume space in both the installation and capacity for installation (empty expansion slots) and hence more complexity. High-speed network topologies require expensive, high-speed integrated circuits and media, while most computers do not utilise all the bandwidth available.
The blade enclosure provides one or more network buses to which the blade will connect, and either presents these ports individually in a single location (versus one in each computer chassis), or aggregates them into fewer ports, reducing the cost of connecting the individual devices. These may be presented in the chassis itself, or in networking blades.
Storage
While computers typically need hard-disks to store the operating system, application and data for the computer, these are not necessarily required locally. Many storage connection methods (e.g. FireWire, SATA, SCSI, DAS, Fibre Channel and iSCSI) are readily moved outside the server, though not all are used in enterprise-level installations. Implementing these connection interfaces within the computer presents similar challenges to the networking interfaces (indeed iSCSI runs over the network interface), and similarly these can be removed from the blade and presented individually or aggregated either on the chassis or through other blades.
In particular, the ability to boot the blade from a Storage Area Network (SAN) allows for an entirely disk-free blade, resulting in exceptional reliability and space utilisation.
Other blades
Since the blade enclosure provides a standard method for delivering basic services to computer devices, these can be leveraged by other types of devices. Blades providing switching, routing, SAN and fibre-channel access can be inserted into the enclosure to provide these services to all members of the enclosure.
Uses
http://en.wikipedia.org/wiki/Image:Pile_of_IBM_HS20s.jpg

A pile of IBM HS20 blade servers. Each "blade" has two 2.8 GHz Xeon CPUs, two 36 GB Ultra-320 SCSI hard drives and 2 GB RAM.
Blade servers are ideal for specific purposes such as web hosting and cluster computing. Individual blades are typically hot-swappable.
Although blade server technology in theory allows for open, cross-vendor solutions, at this stage of development of the technology, users find there are fewer problems when using blades, racks and blade management tools from the same vendor.
Eventual standardization of the technology might result in more choices for consumers; increasing numbers of third-party software vendors are now entering this growing field.
Blade servers are not, however, the answer to every computing problem. They may best be viewed as a form of productized server farm that borrows from mainframe packaging, cooling, and power supply technology. For large problems, server farms of blade servers are still necessary, and because of blade servers' high power density, can suffer even more acutely from the HVAC problems that affect large conventional server farms.


An IBM bladecenter, with an HS20 server partially removed. The top media tray can be switched between all servers.
History
Complete microcomputers were placed on cards and packaged in standard 19-inch racks in the 1970s soon after the introduction of 8-bit microprocessors. This architecture was used in the industrial process control industry as an alternative to minicomputer control systems. Programs were stored in EPROM on early models and were limited to a single function with a small realtime executive.
The name blade server appeared when cards included small hard disks or flash memory program storage. This allowed complete server operating systems to be packaged on the blade.
The architecture of blade servers is expected to move closer to mainframe architectures. Although current systems act as a cluster of independent computers, future systems may add resource virtualization and higher levels of integration with the operating system to increase reliability.
The first company to produce a blade server was Houston-based RLX Technologies (although not proven), which consisted of mostly former Compaq Computer Corp employees. RLX was later acquired by Hewlett Packard (HP) in 2005.
At present IBM remains the global leader in blade servers in terms of market share and revenue with their BladeCenter system and http://www.blade.org industry collaboration initiative. IBM also supports an Open Architecture called the "Blade Open Specification"
Other major players in the blade server market include Hewlett-Packard (HP), Dell, Rackable (Hybrid Blade) and Verari Systems.