melchiz: Windows Live Essentials is a set of free applications from Microsoft. These include Windows Live Messenger, Writer, Mail, and Photo Gallery (Movie Maker is included but requires a WGA check, so I will exclude it from this discussion). In order to install these applications, users must download a separate tool, called the Windows Live Update Tool. This application allows users to select which programs to install, and then downloads and installs them. Windows Live Essentials are not available as individual downloads. Also, in order to update most of these programs, users must run the Windows Live Update Tool, as direct browser downloads of updates are not available. Downloaded applications cannot be moved to another machine (hacks may allow this) without the use of the Windows Live Update Tool.
The Windows Live Update Tool is not an installed client, though, unlike Impulse. The Windows Live Update Tool is a standalone executable which you can run at any time you want in order to download and install any or all of the Windows Live applications. It is simply a standalone executable installer and does not install some client on the machine. The only installed apps are those apps you choose to install. It also serves no other function.
Also, the Windows Live Update Tool is not bound to any user account and can be used on any machine (though if there are any Windows Genuine Advantage checks then they are the DRM).