Boot image

A boot image is a type of disk image (a computer file containing the complete contents and structure of a storage medium). When it is transferred onto a boot device it allows the associated hardware to boot. 
The "boot image" usually includes the operating system, utilities and diagnostics, as well as boot and data recovery information. It also includes those "applications" used organization-wide. A specialized image for a particular type of user or department is called typically a departmental boot image. Building such an image can take days or weeks, and involve complex decisions about licensing and permissions - including which passwords to store in the boot image and which to require users to type in - and requires experts in software integration to do.
However, once built, the boot image can be simply copied onto devices, patched within reasonable limits, and remains disposable in case of any problems (viruses in particular). This is possible because unlike other hard drive images (which may contain any data, et al.), pure boot images contain no mission-critical data. By definition a pure boot image contains no data that cannot be reproduced from configurations or off-the-shelf executables. In particular end-user data is not part of a boot image, although some operating systems require that a copy of user preferences or configuration files be kept within the boot image itself, e.g. Microsoft Windows registry. Utilities like Norton Ghost keep a backup copy of the boot image, for quick re-imaging (often called re-installation) in the event of a problem, thus avoiding the need to diagnose a specific problem with a specific machine.
As virtual machine.
Some virtual machine infrastructure can directly import and export a boot image for direct installation to "bare metal", i.e. a disk. This is the standard technique for OEMs to install identical copies of an operating system on many identical machines: The boot image is created as a virtual machine and then exported, or created on one disk and then copied via a boot image control infrastructure that also makes virtual machine copies. The VMware vCenter Converter for instance lets users "convert physical machines to virtual machines - for free" as part of that company's suite of products to make images easier to back up and manage. Equivalents exist for Xen and other VM systems.
Goals.
By keeping the boot image entirely separate and disposable, and mandating boot image control, organizations seek to keep their total cost of operations (including its total cost of ownership component) low. Often such organizations look at uptime as a service.
One goal of boot image control is to minimize the number of boot images used by an organization to reduce support costs. It includes at least:
Many organizations use thin clients for applications which require high security, involve unreliable users or repurpose older machines for continued use.
A cascading strategy involves re-imaging older, off-spec machines to thin client boot images so that they may continue in use for some less demanding or more access-controlled applications.