wutz a prefab?

prefab = prefabricatedThis term is wide open to interpretation.
prefab home = "kit home" = "panelized home" = "pre-cut home" = on-site assembly from prefab components.
The components are either generally available when the structure is designed or they are created specifically for the project (i.e. designed for reproduction)

mobile home*
manufactured off-site,transported to the site in a largely completed state.minimal on-site labor.

*mobile home is a technical term for a structure that was constructed before June 15, 1976 and meets certain criteria defined by the United States Department of Housing and Urban Development (HUD). Regulations distinguish a “mobile home” from a “HUD-code manufactured home” which is a structure constructed on or after June 15, 1976 and meeting slightly different criteria than a "mobile home". "mobile home" is frequently used in the vernacular, even by HUD who often refer to "manufactured (mobile) homes". Further info here.Info credit: Kevin Dickson.