UTM, Universal Transverse Mercator, is a system of world coordinates like latitude and longitude, from 80 degrees south latitude to 84 degrees north latitude, except the measurements are in meters and UTM lines are orthogonal (always at right-angles to each other). This projection makes all latitude and longitude lines (except the central meridians) curved.
Unlike the regular Mercator, "Transverse Mercator" is a projection with the cylinders rotated 90 Deg (to east-west) and touch the earth "universally" at 60 "central meridian" longitudes. Coordinates are "UTM northing" and "UTM easting."
UTM Northing is the distance north from the equator in meters and Easting is the distance east from 60 central meridians of 6-degree-wide zones starting at longitude 180 degrees.
UTM Northing is divided into 8-degree-high zones from 80 Deg. south latitude to 84 Deg. north latitude using the letters C thru X with the equator at M/N. A,B and Y,Z are reserved for the UPS coordinate system at the poles. For a map of UTM zones see (HERE).
All UTM coordinates are POSITIVE by incorporating a system of "false" easting and northing. Adding an arbitrary 500,000m to the central meridians and 10,000,000m to the distance south of the equator in the southern hemisphere keeps all readings positive.
The UTM map scale increases to either side of the central meridian, so the scale is reduced to 0.9996. This way there will be two meridians to either side of the central meridian where the scale will be correct which improves the average map scale accuracy.
The original definition of the meter was 1/10,000,000th the distance from the equator to the north pole (which is no longer exact.) Due to the oblate shape of the earth, UTM (WGS-84 datum) northing for 45 Deg. N latitude is 4,986,272m instead of 5,000,000m. For UTM (NAD-27 datum) the northing value is 4,986,055m.
This 217m difference is related to the UTM northing errors of approximately 200m experienced by posters here on the NG with some DRG maps. This is due the program, GPS, or user selecting the wrong datum.
Map of UTM Zones