By routing the keyboard, mouse and monitor signals through to one set of well... keyboard, video and mouse you save a bundle on hardware otherwise necessary to operate two, three, or even four computers. There are KVM's (as they are called) which can handle much more than 4 four computers, but their cost is significantly higher as the complexity of the device increases.
The Belkin Omniview KVM supports up to four computers with monitor resolutions of as high as 2048x1526 @ 85Hz. The unit is flash upgradeable so as firmware improves the unit can be upgraded.
Retailing for about $180USD through Belkin's website, or other online stores, the Belkin OmniView SOHO series 4-port KVM Switch with Audio PS/2 and USB offers users a higher-end solution than those beige media switch boxes which commonly retail for less than $30.