What is the difference between packages and artifacts?
Mainly an artifact is the result of of a build phase, this mean a package is an artifact of a kind.

A package is usually a way to install a software or application, it includes the software itself and some intelligence to setup and configure the software.

Calling an artifact a package usually comes when the artifact (whatever it is, from a .deb to a simple zip file) contain more than just the application, it could be a simple script to help configuring the software and setting proper permissions or other softwares needed.