What should I expect to see on my nuvi 255 display?

Essentially there is no difference. It's how they are used. A waypoint is an intermediate point you want to pass through on the way to your destination. A POI can be used as a waypoint or a destination.

For more information, try reading the FAQs including Getting Started. They should help you understand your unit.

I would also download the owner's manual from the Garmin site. The little getting started booklet isn't really a lot of help except for the most basic of features.

Favorites and Waypoints are different names for the same thing, newer models call them Favorites. These are locations that you created and stored directly into the device apart from POI. They appear on my nüvi's screen at zoom levels only below 2 miles. POI (Custom or factory) won't appear on the map until I am zoomed in to 300 feet.

The rationale for this is that you will have relatively few Favorites as many models store a maximum of 500. The factory installed POI number at least 4 million and the number of Custom POI that you can add is up to you. If the POI were visible at the lower zoom levels you might not be able to see the roads!

If your 255 is similar to my 750, then you will see icons for Favorites as you drive if you are zoomed in at 1.2 miles or closer and POI only if you are at 300 feet or less.

As a_user stated, Favorites and POI are functionally the same; you use either to set a destination or a via en route to a destination. Favorites have no particular organization - they are simply a list of your favorite spots. POI are organized into groups so that you can find a Post Office or a Cracker Barrel.