Packer

Official docs outline

Terminology and Concepts
Builders, provisioners, post-processors, artifacts, images and templates.
HCL Templates
Packer template syntax, blocks, and file structure.
Sources and Builders
Builders/sources that create machine images on each platform.
Provisioners
Installing and configuring software in images (shell, file, Ansible).
Post-Processors
Post-build steps such as manifest, compress and checksum.
Variables and Locals
Input variables, locals, and variable files.
Functions and Expressions
Built-in functions and expressions for templates.
Plugins
Installing and using plugins for builders and provisioners.
Data Sources
Fetching external data for use in templates.
Commands
init, validate, build, fmt and CLI usage.
Debugging
Debug mode, logging and troubleshooting builds.
HCP Packer
Image registry, channels, and metadata tracking.

Each section links to the official page. Titles and summaries by Extra Credit; the docs belong to their authors.