System properties are set on the Java command line using the -Dpropertyname=value syntax. They can also be added at runtime
using System.setProperty(String key, String value) or via the various
System.getProperties().load() methods.
To get a specific system property you can use System.getProperty(String key) or System.getProperty(String key, String def).
Environment variables are set in the OS, e.g. in Linux export HOME=/Users/myusername or on Windows SET WINDIR=C:\Windows etc,
and, unlike properties, may not be set at runtime.
To get a specific environment variable you can use System.getenv(String name).