The code of Windows 7 and its predecessor Microsoft Windows Vista is made up of a language independent core, and user interface resources which are responsible for displaying your Windows in English, Greek and Bengali (Bangladesh), for example. Separating the two components has a number of advantages for software manufactures, system administrator and users alike. A Windows 7 Ultimate based computer can, for instance, be configured to display the GUI depending on the language preference of the user currently logged on. However, Windows 7 in English, Greek and Bengali have a completely different architecture. From the user experience perspective this manifests itself in the extent of localization (EOL).
To understand Windows 7 EOL we quickly need to have a look at the user interface categories. The Core UI is the part of the Windows 7 interface which all users use, e.g. the desktop. The Most commonly-used UI being used is all the majority of Windows 7 users see in the lifetime of the computer and includes components such as disk management. The Remaining UI most Windows 7 users will never see, and includes components like command line executables. So, when we talk about full EOL it means that all three UI components have been localized. Partial EOL denotes a Windows 7 localization comprising the Core UI and the most commonly-used UI; in LIP - ELO only the commonly-used UI is localized.
Any Windows 7 installation has user interface resources such as English (US), yet only the Windows 7 flagship editions Ultimate and Enterprise allow the installation of one or more fully or partially localized language packs on top of the base language. However, every Windows 7 edition including the flagship editions allow you to install one or more Language Interface Packs (LIP) on top of the parent or base language.
To find out which edition of Windows 7 you have please check Bright Hub’s article How to install a Windows 7 Language Pack.
A fully or partially localized language pack is also known as a Multi-User Interface (MUI) Pack or just Language Pack (LP). MUIs require a license. A LIP, in contrast, is free. Below are three graphical scenarios  involving language packs: