www.rambler-info.org.uk - GPS data format and datum
"Map Datum" and "Position Format" are "Garmin speak"
you can find other wording elsewhere.
That is the question. Let me give two answers and then you can take it on from there, if you really want to.
Set the Datum to WGS84 and the Data Format to UTM/UPS. Or, more realistically, just set the Data Format to UTM on your GPS and you will find that the Datum has now been set WGS84 automatically.
WGS84 is a set of information for describing the shape of the Earth in terms that let a computer plot its stored data. It assumes that the Earth is a slightly flattened sphere and has a slightly distorted surface to allow for heavy minerals below the surface. It also puts some reference marks on it, so we know where to start when placing our data on the blank shape.
Why UTM? It is a good way of doing things and avoids the horrible 360 and 60 divisions of Latitude and Longitude measured in degrees, minutes and seconds. A position is reported using a format rather like the Ordnance Survey grid references, familiar to all UK ramblers, but using more letters and numbers. (UPS can be forgotten, unless you plan to walk near the Earth's North and South Poles)
You have absolutely no choice here: you must look on the map to see what Map Datum the map makers have used and you must set the same one. Then you should set the Position Format to the one they use. If you do not set the right datum, your GPS may misplace you on the map. The error could exceed 100 m in the worst cases.
Why do different map makers not all use the same system? There are lots of reasons. They do not want Continental Drift sliding the countryside past their national grid for a start. (Even so, if they are Indian then they cannot escape the fact that the south of their country is burrowing under the north).
In countries which would be split by a UTM boundary, they would prefer that the adjacent maps in their range fit together properly at the edges. This is easily done by using an offset system so that none of the UTM boundaries cut through you country. e.g. Spain
The good news is that your GPS will store the data using the WGS84 datum and then calculate the map data from it as needed.
The bad news is that there are software authors around who permit storage of data (on paper or in computer files) using local datums and some of these authors have not built-in proper precautions to prevent users from inadvertently reading it back while assuming a different datum.
There is plenty of information on the Internet, but the above site has some of the clearer information and it a particularly useful one for UK users.
For our present purposes Page 4 and Page 5 are the most relevant entry points.
Their home page is at