JD4x4's test file
First I'll say that I have both a Nüvi 200W and a 765T and I use Windows XP and Garmin's POI Loader to get the POI's into them. When I edit I use a combination of Notepad, Excel, GeePeeEx editor, and Extra POI Editor, usually depending on whether it's a CSV or GPX file and how many items I want to edit.
There are a LOT of great FAQ's and message replies on this site, but unless you know how YOUR particular Garmin device displays you'll be scratching your head for quite a while like I did.
I've created a compiled (run through POI Loader) .gpi file that you can put in your unit's poi folder so you can see how yours displays.
The test file is made up of 3 "sets" of POI files. I call them "sets" because while they actually only contain one "POI" each, they could just as well contain multiple POIs and if they did you will see that the device groups them first by the name of the file that was processed by POI Loader and then by each POI's "Name". The POI name is an actual field in a GPX file and in a CSV file the 3rd column is used for the same purpose. I used 3 different files (each containing one POI) to show that the information displayed is formatted a little differently between a CSV file and a GPX file, and if a GPX file has information in its address fields.
I've put the words "CSV Col 3 (POI Name)" as the first "line" in the csv sample POI, and "Comment" and "Description" as the first lines in the sample GPX POI in the fields Cmt and Desc so you can see where they show up on your device. Then the first lines end with an "end of line character" and are followed by multiple lines, each having a different character as its end of line marker. I used some combinations that the various POI processing software may never use just for the sake of covering as many variations as could be possible. The usual eol (end of line) characters are the html (web page language) expressed as "" (without the quote marks), the hex characters 0D followed by 0A which are called "carriage return" followed by a "line feed", and some various combinations of these. Think of a typewriter.. when you wanted to start a new line the "carriage" with the paper moved so you would start typing at the left side (carriage return), and the paper advanced upward (line feed). Computers don't need to do that and so various eol combinations have evolved. The two that you will most likely run across act differently between these two basic types of devices (the 200W and 765T) and they are and 0D0A, and sometimes just 0A, so you should pay particular attention to these.
Once you see the differences between how each device displays end of line characters, then you can start to explore the differences between CSV and GPX formats. You can do it the other way 'round if you like but for me understanding the end of line characters and how they show up on your device was my first step before seeing the difference between CSV & GPX.