Packer is an open source tool for creating machine images for multiple platforms from a single source configuration. It uses templates that define builders, provisioners, and post-processors to automate the creation of machine images in parallel. Templates use JSON and allow variables, functions, and conditionals. Common builders include Amazon EC2, Docker, Azure, and more. Provisioners like shell, Ansible, Chef, and Puppet install and configure software. Post-processors perform tasks like uploading, compressing, or tagging the finished image.