Artifacts: result of a build. May be one or multiple files and IDs that constiture an image.
Builds: process of creating an image for a given platform.
Builders: component of Packer that is able to create image for a given platform. There is a builder for each "supported" platform.
Commands: options of the Packer executable.
packer build is a command.
Post-processors: component of Packer that permit to run an action on artifacts: compress, upload, etc...
Provisionners: component of packer that install/configures softwares in a running machine that will be turned into an image. Ex of provisionners: scripts, chef, puppet...
Templates: JSON file which defines one or more builds.