Wikipedia
Xserve was a line of rack unit computers designed by Apple Inc. for use as servers. When the Xserve was introduced in 2002, it was Apple's first designated server hardware design since the Apple Network Server in 1996. In the meantime, ordinary Power Macintosh G3 and G4 models were rebranded as Macintosh Server G3 and Macintosh Server G4 with some alterations to the hardware, such as added Gigabit Ethernet cards, UltraWide SCSI cards, extra large and fast hard drives etc. and shipped with Mac OS X Server software. The Xserve initially featured one or two processors, but was later switched over to the then new and subsequently switched again to two quad-core microprocessors.
The Xserve can be used for a variety of applications, including file server, web server or even high-performance computing applications using clustering – a dedicated cluster Xserve, the Xserve Cluster Node, without a video card and optical drives was also available. On November 5, 2010, Apple announced that the Xserve line would be discontinued on January 31, 2011 and replaced with the Mac Pro Server and the Mac Mini Server.