Packer is an open source tool for creating identical machine images for
multiple platforms from a single source configuration.  Packer is
lightweight, runs on every major operating system, and is highly
performant, creating machine images for multiple platforms in parallel.
Packer does not replace configuration management like Chef or Puppet.
In fact, when building images, Packer is able to use tools like Chef or
Puppet to install software onto the image.

A machine image is a single static unit that contains a pre-configured
operating system and installed software which is used to quickly create
new running machines.  Machine image formats change for each platform.
Some examples include AMIs for EC2, VMDK/VMX files for VMware, OVF
exports for VirtualBox, etc.
