A blade server is a server chassis housing multiple thin, modular electronic circuit boards, known as server blades. Each blade is a server in its own right, often dedicated to a single application. The blades are literally servers on a card, containing processors, memory, integrated network controllers, an optional fiber channel host bus adaptor (HBA) and other input/output (IO) ports.
IBM BladeCenter - The Right Choice
A blade server is sometimes referred to as a high-density server and is typically used in a clustering of servers that are dedicated to a single task, such as:
File sharing Web page serving and caching SSL encrypting of Web communication The transcoding of Web page content for smaller displays Streaming audio and video content