Datum- First of all you must understand the earth isn' t round! It' s lumpy and ellipticle. To adjust for this imperfect world, datums are used. Datums are way to adjust to the lumps so points can be plotted. Bottom line- be sure that your GPS and maps use the same system to adjust for the lumps/datum. If not your buried treasure that you plotted in your backyard might read it' s in the middle of the river.

UTM- Instead of using degrees/min/sec to reference a point which is about as useful to me as a rubber hammer and confusing as French Film Noir, instead UTM assigns it a number.
example: 13T265000mE. and 4477000mN.
The first part of the number ( the hard part) is the ZONE ( like time zones), in this case Zone 13T. Because of the mapping lines that converge as they get near the Poles. A line is drawed in the middle of ALL the different zones and is assigned the arbitrary number 500,000. The beginning and ending number of the zone will CORRESPONDINGLY BE LESS or MORE DEPENDING ON FAR NORTH or SOUTH YOU ARE, due to the converging lines, as the Zones become wider or narrower. As you head east the number will increase until it gets to the start of the next zone and your GPS UTM reading will go from something like 13T654123mE to 14T247005mE, past the 500000m line until the next Zone .

The second number 4477000mN ( the real easy part) tells me that it' s 4,477,000 meters North of the equator.

For more detail on this see: www.dbartlett.com/