The Purpose of Language Packs
Vista language packs provide localized Windows interfaces (e.g. Explorer, Desktop and Control Panel) on top of the language-neutral operating system. All Vista editions contain at least one Fully Localized Language Pack which is the (primary) installed language such as English (United States) for example.
A Partially Localized Language Pack has less than 100% of its interface translated and sits on a Base Language Pack, a fully localized language pack which provides translations absent in the former. For instance, the Greek partial language pack depends on the base language pack English (United States) - there is no fully localized language pack in Greek for Vista.
A Language Interface Pack (LIP) allows for partial, commonly used items-localization of the Vista interface in a language different to and depending on a Parent Language Pack, a pertinent, fully or partially localized language pack supporting one or more LIPs. For instance, Assamese, a language of India, runs as LIP only on top of English (United States) as parent language supporting seven different LIPs.
A Multilingual User Interface Pack (MUI) allows for a large part of the Vista interface to be localized on top of a partially or fully localized Windows Vista Ultimate or Vista Enterprise language pack. MUIs are also called Windows Vista Ultimate Language Packs.