A List Apart – Faceted Navigation:

"The distinction between faceted navigation and parametric search is relevant. In parametric search applications, users specify their search parameters up front using a variety of controls such as checkboxes, pull-downs, and sliders to construct what effectively is an advanced Boolean query. Unfortunately, it’s hard for users to set several parameters at once, especially since many combinations will produce zero results. […] It’s a solution that’s hard on people but soft on hardware. In other words, it’s an unfortunate compromise that sacrifices immediate response to reduce the server load."