A Blade is a self-contained densely packed server which fits into an enclosure (or chassis). Compared to a standard 1U, 2U or 4U rack mounted server they only contain the most essential components needed and are very space efficient. Each Blade contains small form-factor hot-swappable drives, Memory, CPU, remote management and multi function I/O interconnects (Power, Network, Management, etc). Blades slide into a slot in a Blade enclosure which connects to the Blade via it's Backplane. Power and cooling is supplied by the Blade Enclosure which allows the Blade to be as compact as possible. Shared storage can be provided by Storage Blades or separate SAN's outside the Chassis, connected via the chassis interconnects.
HP BL460c Gen 9 Blade
The Blade enclosure provides the power, cooling and network requirements for each individual blade.
The biggest benefit of having Blades and a Blade Chassis/Enclosure vs a large rack full of servers is space and energy savings. A HP BladeSystem c7000 can hold 16 Blades and is only 10u in size and fits into a standard rack. Blades can provide a significant saving in a DataCenter (where space is expensive) and running costs as well as providing a lot of flexibility allowing a reduction in downtime. Power is supplied to the Blade Enclosure, which dynamically controls how much power is consumed and the cooling depending on the demand which saves power.
There are four major Blade solutions on the market. HP BladeSystem, Cisco UCS B-series, Dell M1000e and IBM BladeCenter H.
If you buy any of the Blade solutions from any of the above vendors, remember that you will be locked to that vendor for the life of the Blade solution. Each Blade and other components in the enclosure are completely unique to the Blade enclosure. If you need to expand years down the line, you can only do so directly from the vendor and not from a 3rd party, the modules are very unique and designed specifically for that enclosure. Considering the very steep price (normally upwards of a 6 figure cost) of purchasing the enclosure and Blades, you should be completely confident in your chosen vendor in the first place!