Mike,
Hello & Welcome to the Forum!
Powered versus Non Powered means one has a motor the other does not (a dummy).
If you were running a two locomotive AB lash-up, the A Unit would be your Powered Locomotive,
the B Unit would be your Non Powered Locomotive.
This isn't written in stone, if you could afford to purchase two powered units, both your A&B Unit's
would be powered, requiring more power from your transformer/power supply.
With a dummy in your lash-up, you can get by with a smaller transformer/power supply.
Happy Rails!
Dave